How To Judge Chatoyant Stones

There are many gemstones that can show chatoyant phenomena, but the cat’s eye of a fine chrysoberyl combined with the stone’s excellent hardness and durability designate it as the precious cat’s eye among gems.

In the trade, the term cat’s eye alone will always refer to chrysoberyl unless another species is mentioned as a qualifier such as quartz cat’s eye, tourmaline cat’s eye, diopside cat’s eye, kornerupine cat’s eye, etc.

Chatoyancy is a reflection effect from oriented parallel fibrous inclusions or cavities. Light reflects off these needles and if the stone is cut correctly, this results in a single sharp band of light running across the top of the stone similar to a cat’s eye. The domed surface of the cabochon acts like a lens to focus the eye. The grading of phenomenal stones involves the judging of the phenomenon itself as well as the other components that make up the accurate grading of a gemstone.

There are five factors to consider when judging the quality of a cat’s eye.

- The cat’s eye itself

Is it sharp and intense? Is it wavy or even and well-centered? Does it extend from girdle to girdle, and is it properly orientated? Does the eye run lengthwise? Does it open and close?

- Does the stone have good body color?

Is the body color desirable?

- Does the stone have good transparency?

Is there good transparency without sacrificing the sharpness of the eye? There is a very delicate balance in maintaining transparency, yet not being so transparent that the sharpness of the eye is weakened. The base of highly transparent cabochons is often left unpolished for this very reason.

- Does the stone have good clarity?

Are there imperfections, internal or on the surface that interfere with the eye? Do the imperfections distract the viewer from the eye, or decrease the transparency of the stone?

- Is the cutting good?

Is the cabochon shape pleasing to the eye? Are you paying for excessive weight below the girdle that will not be visible when viewed face-up?

Fracture, Cleavage, Parting

1. What is fracture?
Fracture is an uneven break in a gemstone.

2. What is cleavage?
Cleavage is an uneven break parallel to a crystal face due to weakness in molecular bonding.

3. What is parting?
Parting is a breakage due to structural weakness either caused by inclusions or areas of twinning.

Toughness

1. What is toughness?
Tenacity is defined as toughness. By definition it is a measure of resistance of a gem to breaking or fracturing.

2.What is the relation of hardness and tenacity?
They are not related. Hard stones are not necessarily tough. Diamond, the hardest, cleave while nephrite, which is not hard, is very tough.

Hardness

1. What is hardness?
Hardness is defined as resistance to scratching.

2. What scale do we use to judge it?
The Moh's scale.

3. List the order of increasing hardness on the scale with an example at each level.
Talc(1), gypsum(2), calcite(3), fluorite(4), apatite(5), orthoclase(6), quartz(7), topaz(8), corundum(9), diamond(10).

4. What is considered to be the minimum acceptable hardness for a gemstone?
7. Stones with a hardness less than 7 will scratch more easily so how they are fashioned into jewelry and how they are cared for is important.

Shape and Cutting Style

How to distinguish between shape and cutting style?

Shape is simply the way the girdle is cut ie., round, square, oval etc., while cutting style is how facets are cut on the crown and pavilion.

Tweezer

What are the characteristics of a good tweezer?

A good tweezer should be dull, with non-reflective surface and good spring, with inner groove near tip.

Antique Cut

An antique cut is defined as square or rectangle shape with corners rounded.

Colored Stone and Light Source

Which light source is used to distinguish color? Which light source is used to judge transparency?

To distinguish color use reflected light.

To distinguish transparency use transmitted light.