Many types of gemstones belong to groups or species which share a common crystal structure, if not a common chemical composition. Not every gemstone variety belongs to a group. Many important gems, such as peridot, zircon, topaz and spinel, are unique varieties that don't share properties with any other kind of gem.
Here are some of the best known gemstone groups:
Beryl
The Beryl species is famous for its most notable member, emerald. But the beryl family also includes aquamarine, morganite, golden beryl and bixbite. All of the beryls are aluminum beryllium silicates, and have a hardness of 7.5 to 8 on the Mohs scale. The different varieties of beryl are distinguished by their color, but they differ significantly in their transparency and clarity as well. For example, where emerald tends to be heavily included, aquamarine and golden beryl will usually have excellent transparency and clarity.
Corundum
The corundum species includes two of the most important colored gemstones: ruby and sapphire. Famous for their excellent hardness (9 on the Mohs scale) and rarity, ruby and sapphire share both a common chemical composition (aluminum oxide) and a crystal structure (trigonal). The main difference is color: ruby is red corundum, while all other corundum colors are classified as sapphire.
Feldspar
Feldspar makes up nearly 60% of the earth's crust. Feldspar is a group of minerals distinguished by the presence of aluminum and the silica ion in their chemistry. There are two main subgroups of feldspar that produce gem-quality material: the potassium feldspars and the plagioclases, a series that ranges from calcium to sodium feldspars. Among the well-known feldspar gemstones are moonstone, orthoclase, amazonite, andesine, labradorite and sunstone. Amazonite, moonstone and orthoclase are all potassium feldspars. They have a hardness of 6 to 6.5 on the Mohs scale, and a vitreous luster. Moonstone also exhibits a unique shimmer known as adularescence. Labradorite, andesine and sunstone are plagioclases feldspars. Like the potassium feldspars, they have a hardness of 6 to 6.5, but they tend to have slightly higher refractive indices, ranking between beryl and quartz.
Garnet
The garnets are a large group of differently colored minerals with similar crystal structure and related chemical compositions. Garnet has historically been highly regarded for its very good hardness (7-7.5 on the Mohs scale) and great brilliance. The garnet family includes the inexpensive red garnets, almandine and pyrope, as well as some rarer varieties. The premium garnets include the red-orange spessartite, the green tsavorite and demantoid, and the purple-red rhodolite.
Jade
Jade has been known and treasured for more than 7,000 years. But only in 1863 was it discovered that jade is actually not a single mineral. What was traditionally called jade is in fact two separate and distinct minerals: jadeite and nephrite. Nephrite is the more common form of jade. Nephrite ranges in color from mid to dark green or grey-green, but it can also be white, yellowish or reddish. Nephrite is slightly softer than jadeite; nephrite is 6-6.5 on the Mohs scale, while jadeite is 6.5-7. They have quite different chemical compositions as well: nephrite is a calcium magnesium iron silicate while jadeite is a sodium aluminum silicate. The two minerals also have different densities. Jadeite has a density of 3.30-3.38 while nephrite is less dense at 2.90-3.03. The two varieties of jade even have different crystal structures. While jadeite's structure is an arrangement of grainy crystals, nephrite is made up of fibrous crystals that interlock in a matted texture. These densely packed and interwoven fibers are extremely resistant to fracturing. So while jadeite is the denser and harder jade, nephrite is actually the tougher of the two.
Opal
The opal species has three subgroups: the precious opals, the yellow-red fire opals, and the common opals. The precious opals are distinguished by their unique play of color, a display of rainbow-lie hues which changes with the angle of observation. The fire opals, named for their orange color, usually show no play of color. Most fire opals are milky and turbid, but especially fine qualities are clear and transparent. The common opals are opaque to translucent and display no play of color; colors range from white to yellow to brown. Opals are unusual in that they are amorphous -- having no crystal structure -- and always contain water (from 3 to 30 percent).
Quartz
Quartz is the second most common mineral on earth, making up about 12% of the earth's crust. Only feldspar is more common. There are two different branches of the quartz family. Macrocrystalline quartz includes rock crystal, amethyst, citrine, smoky quartz, rose quartz and tiger's eye. Macrocrystalline quartz, as the name suggests, has large crystals which can be distinguished by the naked eye. This type of quartz is mainly transparent to translucent, with a vitreous luster. The other type of quartz is known as cryptocrystalline quartz. It has microscopically (or submicroscopically) small crystals and is usually translucent to opaque, with a waxy to greasy or dull luster. This kind of quartz has fibrous and granular subcategories. The fibrous varieties are known under the name chalcedony, but this name covers a large variety of stones, including agate, jasper, bloodstone, carnelian and chrysocolla.
Spodumene draws its unusual name from the Greek spodumenos, meaning ash-colored, in reference to the color of non-gem spodumene crystals. Gem-quality spodumene is rather more colorful, ranging from colorless to yellow, pink to violet, and yellowish-green to medium-deep green. But gem-quality spodumene occurs very rarely, and the two of the best known varieties, hiddenite (green) and kunzite (pink), were discovered only in the last 130 years.
Tourmaline
The tourmaline group includes a number of related species and varieties. Some tourmaline are named according to their color, including achroite (colorless), dravite (yellow brown to dark brown), indicolite (blue), rubellite (red), schorl (black), and watermelon (pink/red, green and white). Mineralogy distinguishes a number of tourmaline species according to their chemical composition, including elbaite and liddicoatite. Unicolored tourmaline are rare. Most show different tones in the same crystal, and many display distinct different colors in one crystal. Cat's eye tourmalines occur in pink and green.
Zoisite
The name of this group of gems may be obscure, but a type of zoisite is one of the most popular gems of our time, tanzanite. There are in fact three gemstone members of the zoisite species: tanzanite, thulite and anyolite. By chemical composition, zoisite is a calcium aluminum silicate with a hardness of 6.5 to 7 on the Mohs scale. Thulite is an opaque, massive manganese-rich variety of zoisite that is pink in color. It was first discovered in Norway in 1820 and named after the mythical island of Thule. Thulite is usually cut as cabochons or for carving ornamental objects. Anyolite was first discovered near Longido in Tanzania in 1954. Though considered a variety of zoisite, strictly speaking it is a metamorphic rock composed of inter-grown green zoisite, black hornblende and ruby. Sometimes it is called Ruby-Zoisite, since its chief attraction is the interesting contrast of green zoisite and red ruby. The name Anyolite apparently comes from the Masai anyoli, meaning "green."