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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.

Silver, being whiter than platinum or palladium, is quite suitable for color as a metal in mounting diamonds, but it is seldom used now on account of its ready oxidation. Despite its easy tarnish, it was used in much Victorian jewelry, such as spray brooches and pieces which contained a large number of small stones. A light yellow tint in a stone would be neutralised by platinum mounting, but lumpy stones look leaden and heavy in this metal. The high cost of platinum in recent years has led to the increased use of palladium in jewelry, and this lower priced metal gives excellent results.

Mounted stones lose much of their brightness and luster if grease or dirt is allowed to accumulate behind the settings. A stone must be surface clean to show its full brilliancy. There are various obvious methods of washing away dirt from behind a stone. The use of an old, soft toothbrush, soap (preferably vegetable soap), and warm water will not injure diamonds, but careful washing in dilute sulphuric acid will clean a stone of most of the attached foreign matter. Better still, it may be boiled in an aqueous solution of sodium hydroxide, dried, and then rinsed in pure commercial alcohol. Or it may be boiled either in a strong solution of potassium hydroxide or in a mixture of two-thirds sulphuric acid and one-third nitric acid (aqua fortis). Washing in benzine or gasoline, followed by rinsing in alchol, is satisfactory, but dust and dirt are not entirely eliminated by merely dipping in alcohol.

Black opals should possess an abundance of red flashes, or pin point flashes, to be of first quality. If the reds are mixed with greens, they are also good, but broad, even colors are not so valuable. If the brown matrix is showing, the value of the stone is still lessened. Even smaller pieces of fine black opal are comparatively rare, that is, stones weighing three or four carats, and $140 (£50) a carat is not a high price for fine qualities. One should beware of black opal doublets, which are frequently seen in good class jewelry. They may be just as attractive as the natural stone, but their commercial value is far less.

In white opals, the most usual variety in use, the background is a milky tinge, whitish, bluish, or greenish, but the best specimens show such a wealth of red flashes that this bodyground is hardly seen. Stones which show red and green, and also only green, are considered good in quality, but the play of color must be strong and not reserved to parts of the stone only. Bluish stones are of not much value, and thdse which are almost all milky white without any color relief are unsaleable.

It must be remembered that different markets may not value the same quality stones at just the same price. There may be more demand in New York for a certain size and color of stone, which may give it a higher value than that placed on it in, for example, London. But it can be said that, among diamond dealers, the value of a stone can be assigned to within a very small limit at a given time.
Stones mounted in a piece of jewelry should be examined very closely since a skilful mounter can apparently improve shape and color as well as hide certain faults. Reflected light from the metal will fall on the diamond and influence its appearance. Platinum and palladium are now more often used than gold since the last reflects a yellow color. No metal should be left close behind the culet for this reason, although in old pieces of jewelry diamonds were often entirely enclosed in the setting, apart from the upper part of the stone. Such stones are often improved by resetting, although a “closed in” setting sometimes hides a rough edge, bad shape, or other imperfections. It should be remembered that square cut stones, or those other than the brilliant cut, are difficult to estimate for weight, and even experienced dealers make mistakes. So extra care must be taken in such instances.

An aid to detecting flaws in transparent stones may be furnished by the use of a simple device. If such stones are examined while they are submerged in a colorless liquid of a high refractive index, any flaws which may be present are clearly seen. For this purpose, a glass cell, about six inches long by one inch deep and wide, may be conveniently used. A glass plate, along which a row of holes has been drilled, is placed inside the cell, which should be liquid tight. As the holes are smaller than the stones which are being examined, the latter rest on the surface of the plate. The liquid is then poured over the plate so that the stones are covered. Examination with a microscope will detect flaws fairly easily.

Faults are classified differently by dealers, and the names assigned have only a general significance. Those stones usually classed as V.V.S. are of high quality, showing very slight imperfections detected by an expert with a loupe. Then there is first pique, which is only very slightly spotted, second pique, third pique, slightly imperfect, imperfect, and obvious.
Shades of color also have their various names—blue white (which is very rare), snow white, white, commercial white, silver Cape, Cape, slightly yellow, slightly brown, yellow, brown, etc. It is useful to be able to compare the color of a stone with another which is known, especially in an unfamiliar light. In general, color in a diamond is a greater detraction as far as commercial value is concerned than are internal flaws. When a stone is not white, it is said in the trade to “draw” color.

Turquoises should be blue to be of any value. Bluish green, green, and a dull, dirty green are all inferior qualities. The inclusion of matrix (black or brown) and other foreign matter which is often seen on the surface of a stone is also a detraction. Really fine blue turquoises of size are scarce, yet $2.80 (20/-) a carat would be a high price for such stones. Very good imitations of this stone are made from blue glass, blue china, and blue enamel.

Good quality lapis lazuli is not uncommon, but it is a stone which is not much used in jewelry. One should look for a deep blue, unspoiled by white or gray streaks and patches. The included iron pyrites as a few specks is no great deterrent to appearance or value. There is little else which can be confused with lapis lazuli. Glass, suitably colored is the best imitation, the commercial Swiss lapis being obviously different to the naked eye.

Fine jade (jadeite) is very scarce, although inferior quality pieces abound. These latter are of little value, as is also the greenstone (New Zealand) variety. Fine jade is a deep green, full of life, and almost like an emerald except that it is translucent and not transparent. It should be free from blemishes, which are usually of a white or gray character, and it should contain no cracks. The colored varieties of jadeite, the pinks, mauves, reds, and browns, are generally of a pale shade, and these are not of great value. Fine pieces of jade realize high prices, although their market is a restricted one. Individual collectors and connoisseurs are the chief buyers of such stones.

Although many people seem to expect to see a flawless white stone, it may be said that flaws of some kind are almost always present in diamonds, despite the efforts of the lapidary to hide them and the protestations of the salesman to minimize them. The most common faults are small black spots, and these may be present in otherwise good quality stones. If above the girdle, they are more obvious than if below the girdle, but in all instances, value is lessened thereby. Spotted stones are called pique goods in the trade.
Cracks are frequently present, generally very small internal flaws resembling a short length of hair. Light is reflected from such broken surfaces, and these flaws are thus more easily seen in certain directions. Minute cavities or bubbles, which may be empty or filled with carbonic acid, are also not uncommon, but all these faults will not be easily seen except in bad quality stones. Careful examination in various directions is necessary, using forceps or tweezers, not the hands, to hold the stone.

Then we have defects in cutting and polishing which decrease the value of stones. These defects may have a purpose, such as retaining more weight to the detriment of proper shape, or they may be due to lack of skill on the part of the lapidary. Small, unpolished surfaces may be present, or there may be nicks or chips, these often being present in the girdle. Such defects may be minimized if the stone is set in a clever mount. Chips may be due to careless cutting or, in old stones, unskilful unsetting from a piece of mounted jewelry. The careless stocking of loose stones may show in small surface marks, these being caused by the stones rubbing against each other. This holds for all varieties of gem stones. Surface lines left from the polishing wheels are also sometimes seen on the facets.
In addition to these defects we have, of course, the defects of color, and especially a milky hue which clouds some specimens. This is due to irregularity of composition, and it is generally obvious to the naked eye. Each and all of these faults lessen the value of the specimen, but the degree of depreciation is one of current market values and cannot be judged except by those who handle stones daily.
In well cut modern stones, the proportions and facet angles should be approximately those already noted, and all facets in the same set should be alike in size and shape. Therefore, each large facet of the stone should be patiently examined, both from the back and the front. But it should be realized that very few stones are perfectly flawless. In practice, small differences in shape are often not seen, weight being retained to the detriment of good proportion.

We will conclude with a list of stones which are to be found in modern jewelry. All other stones not mentioned in this list are mainly of academic interest only.
Diamond.
Sapphire (mainly blue), and star stones. Green and purple are uncommon, but white used in cheaper forms of jewelry.
Emerald.
Ruby  (star ruby uncommon).
Pearl  (real and cultured).
Opal  (all varieties, the least used being fire opal).
Aquamarine.
Tourmaline  (mainly green and pink).
Spinel (all colors, but mainly red and blue; not greatly used).
Topaz   (mainly yellow, some pink).
Zircon (blue, white, some yellow and brown).
Chrysoberyl   (all fairly uncommon).
Garnet (mainly pyrope and almandine; hessonites uncommon, demantoid rare).
Kunzite  (rare in Europe).
Sphene   (rare; usually brownish yellow in color).
Peridot (usually green).
Beryl (mainly pale and intermediate shades).
Quartz (most varieties are used, but especially amethyst, carnelian, and rock crystal).
Turquoise.
Lapis lazuli.
Jade (jadeite is more common than nephrite).
Moonstone.
Amazonite.
Labradorite.
Malachite.
Fluorspar  (mainly Blue John in England).
Steatite (soapstone).
Also amber, coral, jet, ivory, bone.

When considering rubies and sapphires, including the star stones, the prevalence of synthetics should be borne in mind, and large and apparently perfect stones should be very critically examined. Do not assume that all the rubies or sapphires in a piece of jewelry which contains many stones are real. Some may have been replaced by pastes or synthetics, and new pieces of jewelry containing diamonds and synthetic colored stones are not uncommon. Star stones are also simulated by star quartz suitably backed, especially in larger sizes. Stones in closed settings should not be bought if the amount involved is large, unless one is prepared to gamble. They may be doublets or synthetics, despite their apparent age or history. Rubies are relatively heavier than diamonds, but as they are often cut in forms other than the brilliant, their weights should be estimated with caution. Actual weighing by a balance is the surest means.

With emeralds, depth of color with brilliancy and the absence of flaws are the chief features to look for. A deep green, showing brilliancy, is the best, but stones are too often pale. Sometimes this is caused by the specimen being cut too shallow for want of sufficient material. Other stones are too dark, which also detracts from their value. Surface marks due to wear may be eliminated by careful polishing, but the lapidary cannot eradicate internal flaws. Such internal fissures and spots are most common in emeralds; in fact, a flawless stone is an exception, and some of the better specimens which reach $1,400 (£500) a carat and more are not entirely free of small flaws. A really flawless emerald of three carats or more might fetch a very high price  (they have sold for
$5,600 (£2,000) a carat), so it is as well to examine such stones very carefully when they appear. But the color must be a brilliant velvety deep green to make the stone of first grade.