The hues and tints of spinel show subtle variations that are matched only by those of tourmaline. Unlike tourmaline, however, spinel may be bright ruby red. The cut stones curving around two pieces of rough from Burma weigh (left to right) 30 carats (Ceylon), 34 carats (Burma), 36 carats (Burma), 30 carats (Ceylon), and 22 carats (Ceylon). (Three-fourths actual size.)
Spinel is an oxide of magnesium and aluminum, and it is not related to ruby. However, because its hardness (8) is only slightly less than that of ruby and its brilliance is about equal to that of ruby, spinel makes an excellent substitute for that gem. Also, because it is more plentiful, spinel costs much less. It is interesting that red spinel, like ruby, gets its color from the presence of traces of chromium.
Synthetic blue spinel is widely used as a substitute for aquamarine, and synthetic spinels of other colors are used as substitutes for many gems. However, the synthetic stones are not ordinarily made in the subtle shades so characteristic of natural spinel. Completely colorless spinel apparently exists only as a synthetic material. Actually, because of its hardness, durability, and many attractive colors, spinel makes a fine gemstone in its own right.
Like ruby and several other gemstones, spinel is found chiefly in the gem gravels of Ceylon, Burma, and Thailand. Appreciable amounts of spinel occur in the Ceylon gem gravels as worn, rounded pebbles of many colors. In the Burmese gravel deposits the spinel is often found as well-formed octahedral crystals. Near Moguk, in Burma, spinel has been found in its original position in the limestone rocks as well as in the eroded stream deposits.
VARIETIES Almandine spinel: Purplish red Rubicelle: Orange-red Balas ruby: Rose red Ruby spinel: Deep red Chlorospinel: Translucent grass green Ceylonite or pleonaste: Opaque dark green, brown, or black Picotite or chrome spinel: Translucent dark yellow-brown or green-brown
QUARTZ
(INCLUDES ROCK CRYSTAL, AMETHYST, AND CITRINE)
Few gemstones can compete with quartz for variety of color. Having a hardness of 7 and occurring in many beautiful varieties, only the relative abundance of quartz prevents the species from attaining top rank among gemstones.
The two kinds of quartz, crystalline and cryptocrystalline (fine-grained) quartz, occur in all kinds of mineral deposits throughout the world. Much of this material is suitable for cutting gems.
Colorless crystalline quartz, or rock crystal, makes attractive faceted gems, and it is used as a suitable substitute for diamond and zircon even though it lacks the fire and brilliance of those gemstones. Some very large, flawless crystals of colorless crystalline quartz have been found. The great Warner Crystal Ball, with a diameter of 12⅞ inches and weighing 106¾ pounds, was cut from such a crystal. In addition to the name rock crystal, colorless crystalline quartz appears in the jewelry trade under such names as rhinestone (not to be confused with the glass substitute), Herkimer diamond (from Herkimer County, N. Y.), and Cape May diamond (from Cape May, N. J.).