Plonk Testing

2007: I like the the term plonk testing. Today the concept may be tricky because of the proliferation of sophisticated colored stones, treated, synthetic and assembled stones of all qualities. For the practising gemologists, gem dealers + jewelers the basic tenets are still the same: visual observation + the right approach. This requires knowledge, interpretation skills, Zen-like patience, discipline, analytical skills + hawk's eye to spot the unknown gemstone (s).

(via The Journal of Gemmology, Vol.VII, No.5, January, 1960) A F Farn writes:

My fellow gemologist and friend, Dr E H Rutland, delights in, and is an expert at ‘Plonk Testing’. To ‘plonk’ (an unusual gemological term meaning ‘to go off the deep end’) is to view a gemstone, usually not one of the better known gemstones, which has recently come to hand and to state categorically what the stone is—usually to the delight of the possessor since he is usually armed with details in advance.

A rule of thumb approach and a keen eye backed by knowledge of gemology are the essentials to expertise. Possibly the hardest school is that which backs its judgment by placing not the stone upon the refractometer but its hand in its pocket.

Having attended many lectures and listened to much theory on the subject of gem testing and the rival and relative merits of certain instruments, I am reminded of certain examination questions phrased in the following style:

- What do you consider the equipment necessary to furnish a model laboratory? or
- What are the most important or imperative instruments a gemologist’s needs?

To answer such questions one usually has or shows a marked preference for certain instruments or gives obvious indications of techniques favored by an instructor. Some people are keen on refractometer work, others specific gravity or the spectroscope, and some of course combine the use of each. There are some tests so rudimentary and positive that is needless to go further, which of course admits knowledge of gemology.

Whilst testing by sight is not necessarily accurate, it is usually applied with a background of knowledge and reasoning. I should have stated earlier that perhaps a lens is permitted, but no more. An instance of such testing could be a completely colorless (or white) stone, whichever term is preferred, with one or two minute bubbles, no double refraction discernible but perhaps a slight ‘chatter-marking’—a sure sign of heating of corundum. Therefore the stone would a synthetic corundum. This, of course, is a very obvious instance. There are, doubtless, countless others which will spring to mind. Most gemologists, of course, test stones for enjoyment at leisure; professionals test against the clock and seldom have much time to browse amongst the undoubted beauties of inclusions in Burma or Siam rubies. The latter, though less expensive commercially, are amongst the most beautiful stones from an inclusion point of view. Having tested some tens of thousands of stones I am afraid I do not linger too long in throes of delight on seeing either a Ceylon zircon spectrum or hessonite garnet inclusions. These are merely speedy recognition signs for testing.

Quite recently a parcel of 284 cts of mixed round stones of various colors, and approximately 3500 stones came into my possession. My first reaction was to glance swiftly through the stones spread out on the table on a sheet of white paper and pick out any likely stones, i.e those which looked interesting. Most seemed to me to be the product of Ceylon—later proved correct. Two stones only proved to be of a likely hue and appearance. The remainder I sorted by color into piles of brown hessonite garnets, red/pink Ceylon garnets, pale blue, green and yellow Ceylon sapphires, and an intriguing assorted colored section of zircons of every hue. The colorless portion, thank goodness, was very small—these are always a headache except in this instance.

Having decided the piles on color, the next step was to check by spectroscope. Every pink/red garnet was swiftly pushed into a spot of light from an intensity lamp focused onto the table, and the garnets fed from left to right. With the spectroscope held in the right hand some 600 stones were accurately observed and dealt with in about two hours (having all the same spectrum helped a lot). The zircons came next and the same procedure took place, except that here some had a full Ceylon spectrum and some just a hint of the 6535 Å line, some metamicts. Again this was a speedy test of approximately similar quantities and time. A point of interest here is that the quick focus spot method of scattered light will give a sharp absorption spectrum from a stone of less than one-tenth of a carat whereas by transmitted light through a microscope it would be flooded out and eye would quickly become fatigued. The next parcel, possibly the largest in number, were those ‘plonked’ as hessonites. Here there could be (I hoped) a spessartite or two and the spectroscope came into play once again. None of the stones showed a spectrum, my eye being focused on the blue section end looking for a sign of manganese in the make-up of the stone. There was no need to take the R.I of any of these stones since a quick check of samples by lens and use of corn tongs showed all had the familiar diopside crystal inclusions—the characteristic sign of hessonite. After a time, when one has concentrated solely on one color of gemstone, the eye becomes attuned and exceedingly quick to distinguish any unusual stone. These three groups had reduced the bulk by about 85%.

The next lots were the potential Ceylon sapphires. Knowledge of color shades indicated no synthetics being present, since these latter stones are usually a finer color than their natural counterparts.

Although the spectroscope was speedy in picking out green-blue sapphires, it only gave a hint of chromium being present in the pale shades of blue sapphires. This was interesting to note, since the evidence was also proved by the slight change from pale blue to pale lavender or pink when being transferred from daylight on the table to the artificial light on the focus from the intensity lamp. Although these stones were small, quite a fine bright fluorescent line could be picked up by eye on tilting the spectroscope to the left. This is a useful tip when ‘searching’ for a spectrum, e.g commercial quality Ceylon sapphires do not readily or easily show a 4500 Å line, but tilting the spectroscope will often bring it into view. (In the latter case the spectroscope is tilted to the right). Having hinted perhaps that my intuition was carried out in an atmosphere partial to absorption spectroscopy, I must state the obvious and point out that for a gemologist it is red on the left, blue on the right, when looking through a spectroscope. The pale blue and yellow sapphires together were checked by lens and tongs for chatter-marking and feathers, of which fortunately there was an abundance. Ceylon liquid feathers and two-phase inclusions are a joy in speedy establishment of origin.

The hard core of ‘seeded’ stones now came to be tested. Several mauve/brown stones proved to have the refractive index of natural spinel, two other deep golden brown stones were chrysoberyl, whilst the remainder of colorless stones, were quartz and topaz respectively. Considering its travels and origins I was lucky not to find a single paste or synthetic stone in the parcel.