mygemologists.com

Of the different varieties of quartz, it is only necessary to mention amethyst, since the others are of little value, even when perfect. But amethyst is an attractive stone, which is always in good demand, and fine pieces of size are comparatively scarce. Yet they realize not more than about $8.40 (£3) a carat, and considerably less if not of high quality.
In amethyst, we look for depth of color and brightness. The deep purplish shade is the best, particularly when it is lively and bright. Such stones are generally of Siberian origin. The South American mined stones are often of good color, but they lack the life of the Siberian stones for diamond rings.

Cut stones which are too thick are common, and these are not easy to mount in jewelry. Recutting is not always possible, since this would lighten the color. Stones should be turned over on their tables and viewed through the lower facets; the color may be concentrated in one part of the stone only, and white patches or bands may be apparent. Internal fissures may also be present. These are all faults, which reduce the value of stones.

This blue, which is quite different from the sapphire blue, should be clear and deep, not a pale greenish blue. It is the evidence of green which makes the stone less valuable. Of course, there should be no internal lines or cracks, and the stone should be cut in diamond engagement rings good proportions, without too much depth. Depth will increase the body color, but the stone will weigh more and it will be more difficult to use in jewelry. Good aquamarines of a deep blue color, free from flaws, and well cut, may reach from $28 (£10) to $70 (£25) a carat. But the usual qualities realize about $14 (£5) a carat, and pale stones can be bought for a few shillings, or about $2 a carat. A fine aquamarine is a beautiful stone, and although difficult to obtain, it is often seen in large sizes.

With most of the colored stones, the depth of body color is the chief factor which makes for commercial value. That is to say, a deep blue turquoise is more valuable than the greenish blue stone, the deep blue lapis lazuli is better than the paler or streaked specimens, and the rich, deep purple amethyst is the shade which obtains the highest price. Internal and external flaws of 3 stone engagement rings, of course, must be absent, and size is important. With these points in mind, we can deal with the remaining well known jewel stones very quickly.
Tourmalines. Green stones should not be too dark nor too pale, and this applies to the red stones. Flawed material is not often cut or used in jewelry.

With the transparent chrysoberyl, the depth of color is most important. Size is also a great factor in determining value, for specimens of five carats and over are not common. The more valuable variety is the alexandrite, and such sizes of this diamond  rings stone, if of good quality, would easily reach $280 (£100) a carat. But the chief property which they must possess is their ability to change their colors distinctly in natural and artificial lights. The twin colors should be greenish (but not too dark a green) in ordinary light, and a raspberry red or a purplish red by artificial light. These twin colors are sometimes seen in ordinary light alone, Small stones of about one carat do not realize a high price—per haps $28 (£10) to $70 (£25) a carat. It is size and color changt which make this stone valuable, and such specimens are very rare and consequently in constant demand.

Aquamarines are a popular stone, but good blue specimens attain a price which preclude their sale in medium priced jewelry. The depth of blue should be carefully considered, for shades vary considerably, and the very pale stones diamond eternity rings have almost no value. If possible, specimens of different qualities should be compared for shade, since aquamarines range from almost the white to the deep, transparent blue.