Most gemstones are exhausting minerals. Their colors vary from the colorless transparency of diamonds to pure black. Some are mottled or banded. Others reflect light waves in sparkling brilliance.

Like most different minerals, gem minerals are crystalline. The crystals vary in size from huge blocks weighing many tons to tiny specks that may be seen solely underneath a microscope. Rock crystal, a typical gem material, is found in chunks massive enough to be carved into massive vases and plates. The crystals of chalcedony are so tiny that this mineral was long regarded as noncrystalline. Minerals like chalcedony are referred to as cryptocrystalline, a term derived from the Greek word for “hidden crystal.” A terribly few gem materials, as well as opal and turquoise, appear to be amorphous, or without form.

A few gem materials are organic substances or have an animal or vegetable origin. The pearl, for instance, is made from secretions of mollusks. The pearls of nice value come back solely from the pearl oyster. Amber is a fossilized tree resin jet, a compact form of lignite; and coral, the skeletons of tiny sea animals.

There are also manufactured gems. Early in the nineteenth century many experimenters successfully made artificial reproductions of rare stones, like the ruby. In 1902 Auguste Verneuil of Paris established the primary commercially successful method for making such synthetic gems. The chief raw material is very pure, powdered aluminum. This is sifted through an intense oxyhydrogen flame, and also the fused material drops down on a sticklike support. A lump (boule) is gradually engineered up, cooled, and cut.

Pure alumina, or synthetically produced aluminum oxide, yields white sapphire. Mixed with a tiny amount of chromium oxide, it makes synthetic ruby. Different mixtures yield blue, green, rose, or violet sapphires. A variation of the Verneuil method, developed within the United States in World War II, produces slender rods up to thirty inches long. They are particularly helpful in the manufacture of jeweled bearings for precision instruments. Synthetic gems are used principally in industry. They need replaced natural stones within the manufacture of most watches.

Chemically, artificial gems and natural gems are identical. An professional, but, will tell a synthetic gem by physical differences. The thin layers shaped by the fabric as it builds into a boule might be seen under a microscope. Gas bubbles are generally found in artificial gems, whereas natural gems could have bubbles of liquid.

Alternative manufactured types are imitation gems and doublets. For clear imitation gems such as ruby, diamond or sapphire glass is used. It is a very onerous variety known as paste, or strass giving the name “paste” to imitation gems. A real gem can additionally be combined with less valuable material. A thin slice of ruby, for example, may be cemented to a base of red paste or of garnet. The ruby forms the high of the cut stone and the mixture may be a doublet.