mygemologists.com

One should always test a large stone to ensure that it is genuine and not a synthetic material or a doublet. Stones set in jewelry or in backed metal mounts should be removed carefully—emerald is soft and brittle—since they cannot be judged fairly for weight or color when so presented. Poor quality emeralds, those of pale color and fissured, are difficult to sell at $5.60 (£2) a carat, so it must be remembered that there is a vast difference between the prices of inferior and fine quality stones.

Emeralds are comparatively light in weight, so their size should not be over-estimated if it is not possible to resort to direct weighing. Moreover, the brilliant cut is unusual with emeralds and diamond eternity rings; they are almost always trap cut if of any size. Repolishing often improves old stones, and it is sometimes advantageous to re-cut larger stones to obtain better proportions. Heat should not be applied to emeralds; in fact, all stones except the  diamond uternity ringsshould first be taken out of their settings if it is necessary to solder or alter the mount in any way. And this needs the hand of a careful and skilful setter.

These same remarks apply to rubies, although here the general values are greater. Any stone over five carats of fine quality can reach $5600 (£2000) a carat, but it would have to be exceptional in all respects. A deep brilliant red is the ideal color, and it must be remembered that brilliancy and “life” must be combined with the rich, deep red shade. Paler colors reduce the stone in value
immediately and considerably. Pink stones, the most common, are far less valuable than the deep reds. This is important to remember. The color must be bright, and internal cracks and flaws must be absent to make a fine stone. Such conditions are very rare. If the facets are not proportionate, this may be improved by recut-ting. Rubies are generally more faulty than sapphires; they are far rarer, and a fine ruby is a much more expensive stone than a fine sapphire. In general, the Burma stones are the most valuable. Then come the Siam stones, and those whose color is yellow fancy pendants is rather like that of some garnets are the least expensive.

Star rubies must possess the same features as a star sapphire to be good, except that the body color should be red. Again, pink or pale pink are not considered good. The essential quality of a good star stone is the perfection of the star, and the deeply colored background. Star rubies can reach $280 (£100) a carat.

Yellow sapphires are not in common use, but they are to be seen as center stones of larger brooches and pendants. They should be bright, free from flaws, and the color should be a clear, strong yellow. Many stones are much flawed, and these are of very little value. Fancy color diamonds earrings should be carefully distinguished from topaz, although values are about the same.

Cabochon stones are not in great demand, and they are seldom of fine quality. Star sapphires, however, reach $140 (£50) a carat and more if over five carats and if they show a distinct clear star and the background is blue. Their value is greatly reduced if the star is not central and if it is indistinct. The body color must be blue to make the stone of fine quality, and specimens having these essential factors are rare. Generally, the fancy yellow pendants background is grey or too light in color, and these ordinary stones fall in value to only a few shillings per carat.

The value of cat’s-eyes lies in the strength and the position of the moving band, coupled with the body color and, of course, the size of the stone. In these respects, cat’s-eyes may be compared with star stones, and a good specimen can realize $140 (£50) a carat, reaching more if fine.

But to reach such a figure, the specimen must be five carats or more in weight. Oval or round stones are the best for shape, since they can be used in rings and brooches. The lapidary can improve the shape of a stone, and it can also be polished, but it is rarely possible to improve the chatoyant effect. The body color should be a honey yellow, the streak must be normally down the center of the cabochon, and it must be straight and preferably thin. That is, the band should pass through the highest point of the cabochon, and it should not be a band but a line. Included specks of foreign matter or small cracks are faults to be considered.

One must be wary of confusing cat’s-eyes with the quartz variety, which sometimes shows similar qualities. There is a considerable difference in market values between the chrysoberyl cat’s-eye and the quartz tiger-eye. The best markets for cat’s-eyes are in the U.S.A. and the Far East, and it is here where the best specimens are seen. There is little demand for the stone in England, except by the connoisseur, and perhaps by the stone dealer. The latter has foreign markets in view if he is interested, in which event only good quality stones would be considered.

All opals, and particularly white opals, should be judged in a good light. They will appear better under artificial light and to a disadvantage in a dull light. Stones may be cut in any form, and they are often thin. If too thin, they may be difficult to use. Odd shapes will also have some bearing on value; they may need to be re-cut, which involves extra cost, besides loss in weight. Opal, however, has a comparatively low specific gravity, so one should be careful of overestimating weights. Doublets of white opal are not so common as those of the black variety.

Fire opals are not in great demand at present. The best qualities are those having a uniform deep orange red color. Orange yellows and paler stones are of little value. Milky tinges or clouds within the stone are further detractions. The name of this stone is misleading since many think that it refers to an opal showing much “fire.” But it is properly used only in connection with those opals which possess the one color—orange red, or a slight variation of this color, throughout the material.

Water opals are most attractive if fine. But if they are almost transparent and without any play of color, they are valueless. Large, fine stones are rare. Their body is almost transparent, perhaps slightly milky, but the essential is the play of color which seems to come from within the stone, and not from the surface.

Of course, porous stones such as opal and turquoise should not be subjected to this treatment. The cleansing of jewelry and stones is really best left in the hands of the jeweler, who has the necessary solutions and facilities at hand.
Many large stones are recognizable by dealers in the trade if they have passed through their hands. But there is an institute in Paris which not only pursues scientific research in gem stones but also issues certificates to owners of tested stones, showing the individual properties of their specimens. A micro-photograph is also kept, which would identify the stone in case of theft, besides eliminating possible fraud whenever it is resold.

Let us now summarize the factors which determine the value of a diamond, in their order of importance:
1.    Weight  (size).
2.    Color. Pure white is the best.
3.    Flaws. Cracks are more detrimental than spots.
4.    Form of cutting. Modern brilliant cut is the best.
We shall now pass to other jewel stones of commercial importance. With sapphires, the most popular color is blue, and the finest are the deep transparent blues which are neither dark nor pale. Stones should be homogeneous in color and bright. Differences in shade within the one stone detract from the value, and one ought to view a sapphire in the artificial light as well as in daylight. Many specimens change to an amethyst blue by artificial light, and these are not worth so much as the Cashmere blue stones, which retain their color under these different conditions.