Probably the most exciting development in gem cutting since the creation of the round brilliant is a style of gem cutting developed in the 1970's by Idar-Oberstein artist-craftsman extraordinaire, Bernd Munsteiner.
Today, Munsteiner's work has been widely copied and even mass produced. The generic style is known variously as the Munsteiner or Fantasy Cut. Stones display facets, but rather than being flat, they are arranged in v-shaped grooves somewhat similar to the striations on certain crystals. Combinations of flat facets and curved surfaces are also sometimes seen.
To know more about the Master cutter and jewelry designer, visit the following websites at:
www.khulsey.com/jewelry/atelier_munsteiner.html
www.munsteiner-cut.de
Wednesday, January 10, 2007
How To Judge Opal
Color
First of all, define the type of opal (black, semi-black, crystal, white, etc.) you are evaluating.
Questions to ask regarding color:
- Play-of-color and pattern of color (if applicable). What are the predominant colors? Reds, violets and oranges are more valuable than greens and blues. Red is the rarest, and if present, all other colors are possible. If blue is visible face-up, then other colors will not be possible. Also take into account the distribution, contrast and abundance of colors present.
- Grading opal patterns is quite subjective and may depend largely of the viewer's personal taste. Generally the more colors that appear to the viewer the more valuable the stone. Stones showing largely blue flash are least valuable.
-Pinfire, consisting of small dots of color is quite common, and not especially valuable, even if the predominant color is red.
-Flash refers to opals with color patterns that appear suddenly as the stone is moved.
-Harlequin refers to opals with a regular arrangement of similarly-sized flashes of color. The more regular the arrangement, the better. Stones showing the full range of colors in this pattern are exceedingly rare and valuable.
- Extinction : Does the full color range appear over a small viewing angle?
- In other words, is there an angle of extinction, during a 360x rotation, no colors are visible? Another way of describing this is directionality - the most valuable stones do not show any extinction of color.
- Brilliancy : How intense are the colors? Do they stand out from the background? How is the color distributed?
- Consider the amount of potch in comparison with the amount of play-of-color. Although potch provides a background for the play-of-color, we buy opal per carat, and so do not want to purchase opals with excessive amounts of potch (potch to play-of-color ratio).
- Sequence of colors : Red that holds over a wide range of viewing angles is the most desirable. In black opal, a red immediately followed by a royal blue is the rarest and most sought-after color sequence.
Practical Tip: Place the opal on a flat surface at arm's length; then rotate it through 360˚ noting the play-of-color, brilliance, and intensity of colors, symmetry and polish. Then hold the opal up to the light and estimate the ratio of potch to play-of-color.
Fine opals should show:
- Uniform play-of-color through 360º
- Intense, brilliant colors
- No 'extinction'
- Symmetrical cut, and fine degree of polish
Judge other opals from this ideal standpoint.
Clarity
Note the following:
- How transparent is the material? High transparency is sought after in light opal, but may reduce the value of black opal.
- Are any fragments of matrix present? Potch, fragments of host rock, sand or other impurities should not interrupt the play-of-color. Material should be free from blemishes visible from top.
- Are there any cracks and flaws visible? Precious opal can contain around 6-10% water and some opals from certain areas may craze, crack and/or turn opaque. Remember, never buy a cracked opal. The crack will only get worse as time passes.
Cut
In judging cut, look for the symmetry (proportions) and polish (finish). Is the make (cut and proportions) of the cabochon good? How good is the surface polish?
First of all, define the type of opal (black, semi-black, crystal, white, etc.) you are evaluating.
Questions to ask regarding color:
- Play-of-color and pattern of color (if applicable). What are the predominant colors? Reds, violets and oranges are more valuable than greens and blues. Red is the rarest, and if present, all other colors are possible. If blue is visible face-up, then other colors will not be possible. Also take into account the distribution, contrast and abundance of colors present.
- Grading opal patterns is quite subjective and may depend largely of the viewer's personal taste. Generally the more colors that appear to the viewer the more valuable the stone. Stones showing largely blue flash are least valuable.
-Pinfire, consisting of small dots of color is quite common, and not especially valuable, even if the predominant color is red.
-Flash refers to opals with color patterns that appear suddenly as the stone is moved.
-Harlequin refers to opals with a regular arrangement of similarly-sized flashes of color. The more regular the arrangement, the better. Stones showing the full range of colors in this pattern are exceedingly rare and valuable.
- Extinction : Does the full color range appear over a small viewing angle?
- In other words, is there an angle of extinction, during a 360x rotation, no colors are visible? Another way of describing this is directionality - the most valuable stones do not show any extinction of color.
- Brilliancy : How intense are the colors? Do they stand out from the background? How is the color distributed?
- Consider the amount of potch in comparison with the amount of play-of-color. Although potch provides a background for the play-of-color, we buy opal per carat, and so do not want to purchase opals with excessive amounts of potch (potch to play-of-color ratio).
- Sequence of colors : Red that holds over a wide range of viewing angles is the most desirable. In black opal, a red immediately followed by a royal blue is the rarest and most sought-after color sequence.
Practical Tip: Place the opal on a flat surface at arm's length; then rotate it through 360˚ noting the play-of-color, brilliance, and intensity of colors, symmetry and polish. Then hold the opal up to the light and estimate the ratio of potch to play-of-color.
Fine opals should show:
- Uniform play-of-color through 360º
- Intense, brilliant colors
- No 'extinction'
- Symmetrical cut, and fine degree of polish
Judge other opals from this ideal standpoint.
Clarity
Note the following:
- How transparent is the material? High transparency is sought after in light opal, but may reduce the value of black opal.
- Are any fragments of matrix present? Potch, fragments of host rock, sand or other impurities should not interrupt the play-of-color. Material should be free from blemishes visible from top.
- Are there any cracks and flaws visible? Precious opal can contain around 6-10% water and some opals from certain areas may craze, crack and/or turn opaque. Remember, never buy a cracked opal. The crack will only get worse as time passes.
Cut
In judging cut, look for the symmetry (proportions) and polish (finish). Is the make (cut and proportions) of the cabochon good? How good is the surface polish?
Tuesday, January 09, 2007
Refractometer
The instrument in general use for the measurement of refractive index is the refractometer. The refractometer determines the refractive index of a gemstone by using the principle of critical angle of total internal reflection, which is used to measure the refractive indices of various substances.
General information
- Refractive Index (RI) measurement is the most useful and accurate means of testing gemstones.
- It is convenient, non-destructive and relatively quick to carry out providing useful information.
Requirements
- Optical contact.
- Clean, polished surface.
- Contact liquid.
- Refractive index (RI) of the stone under test should be within the range of the instrument.
Limits
- Upper limit of the refractometer is determined by the refractive index of the prism as well as the refractive index of the contact fluid / medium.
- In a normal commercial refractometer, this is 1.81 (some RI liquids may have a limit of 1.79).
Results
- Flat surface readings: single refractive (SR) / double refractive (DR), birefringence, optic character (uniaxial or biaxial), and optic sign (positive or negative).
- Curved surface readings (distant vision method): Mean refractive index only.
General information
- Refractive Index (RI) measurement is the most useful and accurate means of testing gemstones.
- It is convenient, non-destructive and relatively quick to carry out providing useful information.
Requirements
- Optical contact.
- Clean, polished surface.
- Contact liquid.
- Refractive index (RI) of the stone under test should be within the range of the instrument.
Limits
- Upper limit of the refractometer is determined by the refractive index of the prism as well as the refractive index of the contact fluid / medium.
- In a normal commercial refractometer, this is 1.81 (some RI liquids may have a limit of 1.79).
Results
- Flat surface readings: single refractive (SR) / double refractive (DR), birefringence, optic character (uniaxial or biaxial), and optic sign (positive or negative).
- Curved surface readings (distant vision method): Mean refractive index only.
Diamond Cut: STARBURST
The Starburst Cut name is a patent of Louis Glick & Co and U Doppelt & Co, developed 25 years ago by the Louis Glick and U Doppelt companies.
The Starburst is a modified radiant cut. It is a rectangular brilliant cut with 88 or more kite and star shaped facets. Often there are twice as many facets on a Starburst as on a traditional round cut.
The Starburst is a modified radiant cut. It is a rectangular brilliant cut with 88 or more kite and star shaped facets. Often there are twice as many facets on a Starburst as on a traditional round cut.
Tasmanian Sapphire
David Federman writes:
Australia was to sapphire in the 1980s what South Africa was to diamonds for more than a century. Still, no one felt blue when Aussie indigos disappeared under a blanket of Madagascar's dressier blues in the 1990s.
The new African corundum boasted color that begged comparison to that of Sri Lankan goods—even on occasion Kashmir classics. Australia's material, on the other hand, was lucky to inspire comparisons to dark but sometimes handsome Cambodian and Thai stones. No wonder Australia came and went as a corundum colossus and no one shed a tear.
More info @ http://archives.modernjeweler.com/publication/article.jsp?pubId=1&id=193
Australia was to sapphire in the 1980s what South Africa was to diamonds for more than a century. Still, no one felt blue when Aussie indigos disappeared under a blanket of Madagascar's dressier blues in the 1990s.
The new African corundum boasted color that begged comparison to that of Sri Lankan goods—even on occasion Kashmir classics. Australia's material, on the other hand, was lucky to inspire comparisons to dark but sometimes handsome Cambodian and Thai stones. No wonder Australia came and went as a corundum colossus and no one shed a tear.
More info @ http://archives.modernjeweler.com/publication/article.jsp?pubId=1&id=193
The Colored Stone Story
What are colored stones?
Colored stones refer to any gem other than diamond. Some diamonds are colored, but colored diamonds are not considered colored stones.
What are the attributes of colored stones?
A colored stone possesses any of these three key characteristics: beauty, durability and rarity. An ideal colored stone has all three characteristics.
Why do colored stones look the way they do?
A number of factors contribute to the overall appearance of colored stones. These include color, transparency, lustre, brilliance, dispersion, scintillation, clarity and cut.
What is the difference between precious stones and semi-precious stones?
In the past, the term ‘precious stone’ referred to diamond, emerald, ruby and sapphire, while ‘semi-precious stone’ were anything other than these. Then, precious and semi-precious stones indicated relative value – the former being more valuable. Today, these descriptions have little meaning in terms of value, since all colored stones come in a variety of qualities. For example, a low quality ruby may sell for as little as USD1 per carat, while a good quality demantoid garnet may fetch USD1,000 per carat.
Are colored stones durable?
In order to wear well and retain attractiveness colored stones should be durable. Durability includes hardness and toughness. Resistance to scratching is referred to as hardness. Some colored stones are soft and may be easily scratched, thereby losing their attractiveness. Ideally a colored stone should be harder than dust particles, which may be fine particles of quartz (hardness=7) found in the air. Colored stones with hardness greater than quartz include ruby, sapphire, topaz, tourmaline, spinel and garnet. These are the most valuable. Resistance to breakage is referred to as toughness, and this depends on colored stone’s structure. Extremely tough gems are usually aggregates such as jadeite and nephrite.
Is there a relationship between quality and price?
In a perfect world price would directly relate to quality, weight and rarity. The real world is far from perfect. Market factors may have an impact on prices, as does quality.
What is colored stone treatment?
Colored stone treatment is a common term. It describes different ways to improve the beauty of colored stones. Colored stones have been enhanced for thousands of years. The truth is there isn’t enough top quality colored stones being mined to meet demand. Enhancement increases supply, ensuring a constant flow of colored stones which we can afford and enjoy. If in doubt about a colored stone, always consult a reputed gem testing laboratory.
Colored stones refer to any gem other than diamond. Some diamonds are colored, but colored diamonds are not considered colored stones.
What are the attributes of colored stones?
A colored stone possesses any of these three key characteristics: beauty, durability and rarity. An ideal colored stone has all three characteristics.
Why do colored stones look the way they do?
A number of factors contribute to the overall appearance of colored stones. These include color, transparency, lustre, brilliance, dispersion, scintillation, clarity and cut.
What is the difference between precious stones and semi-precious stones?
In the past, the term ‘precious stone’ referred to diamond, emerald, ruby and sapphire, while ‘semi-precious stone’ were anything other than these. Then, precious and semi-precious stones indicated relative value – the former being more valuable. Today, these descriptions have little meaning in terms of value, since all colored stones come in a variety of qualities. For example, a low quality ruby may sell for as little as USD1 per carat, while a good quality demantoid garnet may fetch USD1,000 per carat.
Are colored stones durable?
In order to wear well and retain attractiveness colored stones should be durable. Durability includes hardness and toughness. Resistance to scratching is referred to as hardness. Some colored stones are soft and may be easily scratched, thereby losing their attractiveness. Ideally a colored stone should be harder than dust particles, which may be fine particles of quartz (hardness=7) found in the air. Colored stones with hardness greater than quartz include ruby, sapphire, topaz, tourmaline, spinel and garnet. These are the most valuable. Resistance to breakage is referred to as toughness, and this depends on colored stone’s structure. Extremely tough gems are usually aggregates such as jadeite and nephrite.
Is there a relationship between quality and price?
In a perfect world price would directly relate to quality, weight and rarity. The real world is far from perfect. Market factors may have an impact on prices, as does quality.
What is colored stone treatment?
Colored stone treatment is a common term. It describes different ways to improve the beauty of colored stones. Colored stones have been enhanced for thousands of years. The truth is there isn’t enough top quality colored stones being mined to meet demand. Enhancement increases supply, ensuring a constant flow of colored stones which we can afford and enjoy. If in doubt about a colored stone, always consult a reputed gem testing laboratory.
The Chelsea Color Filter
The Chelsea Color Filter was first developed by the Gem Testing Laboratory of the London Chamber of Commerce (now GAGTL/Gem-A) in 1934, and was first used at the Chelsea College of Science and Technology as an aid in the separation of emerald from its simulants. It was originally called an emerald filter.
The Chelsea filter consists of a combination of two gelatin filters that pass wavelengths in the deep red and in the yellow green. To obtain best results, stones should be viewed under a strong artificial light with the filter held close to the eye so as to cut out any outside glare of light. The gemological uses of this simple instrument are limited, but it can be useful in certain circumstances.
The chromium in emerald makes it appear reddish or pinkish when the stone is strongly illuminated and viewed through the filter. Emerald imitations that do not contain chromium appear green through filter. A few natural emeralds, especially those from South America, fail to show pink through the filter. Once synthetic emeralds appeared on the market, the Chelsea filter became less useful as an emerald filter, because the synthetic product, also containing chromium, appears red through the filter, though often of a deeper red than natural counterparts.
The Chelsea filter is also useful in separating cobalt-containing gem species from others, as the presence of cobalt results in a red reaction through the filter. Gemstones colored by cobalt are in the main synthetic or man-made, and appear red through the filter. This has proven useful in separating synthetic blue spinel and man-made blue glass from the stones they imitate, namely aquamarine, blue zircon and sapphire. Gemstones containing cobalt are rare on earth.
Aquamarine and its most common simulant, synthetic blue spinela are visually very similar, but owe their respective blue color to different causes. Aquamarine is colored by iron, which absorbs red wavelengths and transmits some green wavelengths, and appears green through the filter. Synthetic blue spinel is colored by cobalt which absorbs green wavelength, but transmits red wavelength, appears red through the filter.
The absorption spectrum of aquamarine, colord by iron, and synthetic blue spinel, colored by cobalt, can help us understand why their reactions to the Chelsea filter are so different.
The Chelsea filter consists of a combination of two gelatin filters that pass wavelengths in the deep red and in the yellow green. To obtain best results, stones should be viewed under a strong artificial light with the filter held close to the eye so as to cut out any outside glare of light. The gemological uses of this simple instrument are limited, but it can be useful in certain circumstances.
The chromium in emerald makes it appear reddish or pinkish when the stone is strongly illuminated and viewed through the filter. Emerald imitations that do not contain chromium appear green through filter. A few natural emeralds, especially those from South America, fail to show pink through the filter. Once synthetic emeralds appeared on the market, the Chelsea filter became less useful as an emerald filter, because the synthetic product, also containing chromium, appears red through the filter, though often of a deeper red than natural counterparts.
The Chelsea filter is also useful in separating cobalt-containing gem species from others, as the presence of cobalt results in a red reaction through the filter. Gemstones colored by cobalt are in the main synthetic or man-made, and appear red through the filter. This has proven useful in separating synthetic blue spinel and man-made blue glass from the stones they imitate, namely aquamarine, blue zircon and sapphire. Gemstones containing cobalt are rare on earth.
Aquamarine and its most common simulant, synthetic blue spinela are visually very similar, but owe their respective blue color to different causes. Aquamarine is colored by iron, which absorbs red wavelengths and transmits some green wavelengths, and appears green through the filter. Synthetic blue spinel is colored by cobalt which absorbs green wavelength, but transmits red wavelength, appears red through the filter.
The absorption spectrum of aquamarine, colord by iron, and synthetic blue spinel, colored by cobalt, can help us understand why their reactions to the Chelsea filter are so different.
Monday, January 08, 2007
Tsavorite Garnet
National Jeweler Network writes:
Michael Coach and Associates has announced that it has acquired the largest clean tsavorite in the world. The stone weighs 325.13 carats and will be on display at the American Gem Trade Association GemFair beginning at the end of January in Tucson, Ariz.
The stone was found in the Karo area of Mereloni, Tanzania, in what is believed to be the biggest pocket of tsavorite ever discovered. The rough was found in a mine that goes straight down 120 meters from the surface and follows a reef at a 45-degree angle to a final depth of 180 meters.
Tsavorite is usually found in a host matrix of diopside, zoisite and grossular garnet, and is normally made in small nodules of gem crystal, which are put under enormous pressure. Because the nodules are squeezed and pushed during growth, inclusions are easily formed, making clean crystals something of a rarity.
From the rough, four stones were cut: a 62-carat clean, a 120-carat clean, a 307-carat included and the 325.13-carat clean acquired by Michael Couch and Associates. Asking price on the 325.13-carat stone is $2.2 million.
More info @ http://www.nationaljewelernetwork.com/njn/content_display/colored_stones/e3i359161611ed1086efe5914532f3faa3e
Michael Coach and Associates has announced that it has acquired the largest clean tsavorite in the world. The stone weighs 325.13 carats and will be on display at the American Gem Trade Association GemFair beginning at the end of January in Tucson, Ariz.
The stone was found in the Karo area of Mereloni, Tanzania, in what is believed to be the biggest pocket of tsavorite ever discovered. The rough was found in a mine that goes straight down 120 meters from the surface and follows a reef at a 45-degree angle to a final depth of 180 meters.
Tsavorite is usually found in a host matrix of diopside, zoisite and grossular garnet, and is normally made in small nodules of gem crystal, which are put under enormous pressure. Because the nodules are squeezed and pushed during growth, inclusions are easily formed, making clean crystals something of a rarity.
From the rough, four stones were cut: a 62-carat clean, a 120-carat clean, a 307-carat included and the 325.13-carat clean acquired by Michael Couch and Associates. Asking price on the 325.13-carat stone is $2.2 million.
More info @ http://www.nationaljewelernetwork.com/njn/content_display/colored_stones/e3i359161611ed1086efe5914532f3faa3e
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