Natural gems include all mineral gemstones. The science of gemology groups them by families according to their crystal systems or chemical composition. Some of these are agate, amethyst, beryl, chrysoberyl, corundum, diamond, emerald, garnet, jade, jasper, lapis lazuli, onyx, opal, quartz, ruby, sapphire, topaz, tourmaline, and turquoise.
A small class of natural gems -- all having some mineral content -- are those of animal or plant origin such as amber, coral, jet, and pearl.
The value of a gem lies mainly in their beauty, durability, and rarity. The beauty of a gem largely depends on the gem's optical qualities. The most important are its luster or brightness, color, and fire. The durability of a gem depends on its hardness and toughness. Harness indicates a gem's resistance to being scratched; toughness, its resistance to fracture. A gem's rarity, together with fashion, affects the demand for and cost of a gem.
Gems that have historically been considered the most valuable are commonly called as precious stones and less valuable gems as semiprecious stones. However, in practice, a high-quality semiprecious stone can be more valuable and cost more that a low-quality precious stone. Precious stones include diamonds, rubies, sapphires, emeralds, and opals. Semiprecious stones include zircons, spinel, and amethysts.
Artificial gems are various kinds of imitation gems, including the following;
A number of early peoples believed that gems were drops of dew, raindrops, or even tears, hardened by the sun. Since the sun rises in the east, and is hottest in the tropics, the most prized gems were those that came from India and Arabia.
The list of superstitions once connected with gems is virtually endless. Amber cured disease. Amethyst assured pleasant dreams. Coral was a charm against shipwreck and fire. Ruby prevented plague, and purified one's thoughts. Stones of all kinds were said to change color as a sign of changing fortunes, or to indicate the death of the distant beloved.
Beyond superstition, yet perhaps arising from it, is sybolism. The great religions, in their early rituals, assigned special meaning to gems. Jade is associated with Buddhist worship. In Old Testiment times, 12 kinds of gems, representing the Twelve Tribes of Israel, were worn in the breastplate of the high priest of Judaism. Among Christians the Twelve Apostles have been represented symbolically by precious stones. Pope Innocent III commanded his bishops to wear sapphire rings as a sign of peace.
In a more popular sense the language of gems endures in birthstones. Different gems are associated with the twelve months of the year. Birthstone lists do not always agree. The following birthstone list, showing cut and polished gems together with their forms in nature, uses the list of the American Gem Society and the Jewelry Industry Council. The list also recognizes, in addition to the gems pictured, alternate gems for March (bloodstone), June (pearl and alexandrite), August (peridot), October (tourmaline), and December (zircon).
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