Some gemstones are minerals, rocks or petrified materials that, when cut or faceted and polished, are collectible or can be used in jewelry. Others are organic, such as amber, which is fossilized tree resin. Gemologists base a gemstone’s category on chemical composition or crystal classification. For example, diamonds are made of carbon, rubies of aluminum oxide, and so forth. Another category is the habitat in which the gem is usually found. For example diamonds, which have a cubic crystal system, are often found as octahedrons. Finally, gemstones are classified into different groups, species, and varieties. For example, ruby is the red variety of the species corundum, while any other color of corundum is considered sapphire.

AMETHYST: “Amethyst” is derived from the Greek word "amethystos", meaning "not drunken." Because it was considered to be a strong antidote against drunkenness, wine goblets were often carved from crystal. In Greek mythology, when a drunken Dionysus was pursuing a maiden called Amethystos, who refused his affections, she prayed to the gods to remain chaste. The goddess Artemis granted the prayer, transforming her into a white stone. Humbled by Amethystos' desire to remain chaste, Dionysus poured wine over the stone she had become as an offering, dying the crystals purple. Beads of amethyst are found in Anglo-Saxon graves in England. It is a widely distributed mineral, but fine, clear specimens suitable for cutting as ornamental stones are confined to comparatively few localities. Such crystals occur naturally either in the cavities of mineral-veins and in granite rocks, or as a lining in agate geodes. On exposure to heat, amethyst generally becomes yellow, and much of the citrine, cairngorm or yellow quartz of jewelry is called "burnt amethyst".


AQUAMARINE: Aquamarine (Latin for aqua marina, "water of the sea") is a gemstone-quality transparent variety of beryl, having a delicate blue or turquoise color suggestive of the tint of seawater. It's closely related to the emerald. Colors vary from yellow beryl (heliodor) to rose pink beryl (morganite) to white beryl (goshenite). People in the Middle Ages believed aquamarine could overcome the effects of poison. Ancient sailors traveled with aquamarine crystals, believing that it would ensure a safe passage, and often slept with the stones under their pillow to ensure sound sleep. And, they believed the mermaid’s fish-like lower body was made of aquamarine.


CHRYSOPRASE: Chrysoprase is the gemstone variety of chalcedony, a fibrous form of quartz that contains small quantities of nickel. Its color is normally apple green, but can be deep green. Due to its comparative scarcity and pleasing green color, chrysoprase is one of the most prized varieties of quartz. Higher quality specimens often rival fine jade, for which it is sometimes mistaken. Cut into cabochons (smooth domed gems with flat backs for use in jewelry), it can be as sought after as fine amethyst. Unlike the emerald, which owes its beautiful green color to the presence of chromium, the color of chrysoprase is due to trace amounts of nickel in the structure.


DIAMOND: Diamonds are believed to have been first noticed in India’s mines where sediment deposits of the stone could be found. The earliest written reference is found in the Anguttara Nikaya, a Buddhist text. A Sanskrit text, the Arthashastra, which was completed around 296 BCE, describes diamond's hardness, luster and dispersion. Diamonds quickly became associated with divinity and used to decorate religious icons, and were believed to bring good fortune to those who carried them. Ownership was restricted among various castes by color, with only kings allowed to own all colors of the diamond. The word “diamond” derives from the ancient Greek "adamas", invincible. Treasured as a gemstone since its use as a religious icon in India at least 2,500 years ago, the diamond’s use drill bits and engraving tools also dates to early human history. Popularity of diamonds has risen since the 19th century because of increased supply, improved cutting and polishing techniques, growth in the world economy, and innovative and successful advertising campaigns.

They are commonly assessed according to the “4 Cs” - carat, clarity, color and cut. Although synthetic diamonds are produced each year at nearly four times the rate of natural diamonds, the vast majority of synthetic diamonds produced are small imperfect diamonds suitable only for industrial-grade use. Diamonds can be found in nearly any color, though yellow and brown are the most common. "Black" diamonds are not truly black, but rather contain numerous dark inclusions that give the gems their dark appearance. Diamonds with a detectable hue other than yellow or brown are known as colored diamonds. If the color is strong enough, a stone may be referred to as a fancy colored diamond by the gem trade. Colored diamonds contain impurities or structural defects that cause the coloration, while pure or nearly pure diamonds are transparent and colorless.


EMERALD: Emeralds come in many shades of green and bluish-green. There is a wide spectrum of clarity, along with various numbers of inclusions, which are foreign minerals or substances inside the gem that can be valuable. A rare type of emerald known as the trapiche emerald is occasionally found in the mines of Colombia. A trapiche emerald exhibits a "star" pattern, which has ray-like spokes of dark carbon impurities that a six-pointed radial pattern. Most emeralds are highly included, so it is quite rare to find an emerald with only minor inclusions. The Hebrew word "nofech" (Exodus 28:18) means "emerald", which was the stone on the Ephod that represented the tribe of Judah. According to other commentaries, "nofech" means "garnet". Another stone, the bareqet, which represented the tribe of Levi, is thought to be an emerald. In some cultures, the emerald is the traditional gift for the 55th wedding anniversary. It is also used as a 20th and 35th wedding anniversary stone.


GARNET: Garnet is a group of minerals that have been used since the Bronze Age as gemstones and abrasives. Garnets are most often seen in red, but are available in a wide variety of colors spanning the entire spectrum. The name "garnet" comes from the Latin “granatus” ("grain"), which is a possible reference to the “malum granatum” ("pomegranate"), a plant with red seeds similar in shape, size, and color to some garnet crystals. Six common varieties of garnet are recognized based on their chemical composition - pyrope, almandine or carbuncle, spessartite, grossularite (varieties of which are hessonite or cinnamon-stone and tsavorite), uvarovite and andradite. The most rare variety is the blue garnet, which was discovered in the late 1990s in Bekily, Madagascar.


JADE: The word “jade” refers to two different rocks that are made up of different silicate minerals. Nephrite jade consists of the calcium- and magnesium-rich amphibole mineral actinolite. The rock called jadeitite consists almost entirely of jadeite, a sodium- and aluminum-rich pyroxene. Nephrite can be found in a creamy white form (known in China as "mutton fat" jade) as well as in a variety of green colors, while jadeitite has more color variations, including dazzling blue, lavender-mauve, pink and emerald-green. Of the two, jadeitite is more rare.


PEARL: A pearl is not a mineral as most gemstones, but rather a hard, rounded object produced primarily by mollusks such as oysters. Pearls can be used in jewelry or crushed for cosmetics or paint. Cultivated or harvested for jewelry, pearls fit into two categories -- freshwater and saltwater. As their name implies, freshwater pearls are formed in freshwater mussels that live in lakes, rivers, ponds and other bodies of fresh water. Most freshwater cultured pearls sold today come from China. By contrast, saltwater pearls grow in oysters that live in the ocean, usually in protected lagoons.

The value of the pearl is determined by a combination of the luster, color, size, lack of surface flaw, and symmetry. Among those attributes, luster is the most important measurement of pearl quality, however, the larger the pearl the greater its value. Large, perfectly round pearls are rare and highly valued. Teardrop-shaped pearls are often used in pendants. Irregular shaped pearls are often used in necklaces.


PERIDOT: The word “peridot” is derived from either the Arabic word "faridat", meaning "gem", or the French word “peritot”, meaning "unclear." One of the few gemstones available in only one color, the depth of the green depends upon how much iron the crystal contains, varying from yellow-green to olive to brownish-green. According to folklore, the peridot will bring its wearer success, peace and good luck. Found in Egyptian jewelry from the early second millennium BC, the peridot was mined from the volcanic island of Zebirget, or St. Johns Island, in the Red Sea. Native Hawaiians referred to peridot crystals as the “Tears of Pele”, their goddess of fire.


ONYX: Onyx is a banded variety of chalcedony, a cryptocrystalline form of quartz, with bands in white and black. Sardonyx is a variant in which the colored bands are sard (shades of red) rather than black. It is usually cut as a cabochon or into beads, and is also used for intaglios and cameos, where the bands make the image contrast with the ground. Some onyx is natural, but much is produced by the staining of agate. The name has sometimes been incorrectly used to label other banded lapidary materials, such as banded calcite found in Mexico and Pakistan. This material is much softer than true onyx and much more readily available. The majority of carved items sold as "onyx" today are made of this carbonate material.


OPAL: Besides the gemstone varieties that show a play of color, there are other kinds of common opal, including the:

• milk opal, milky bluish to greenish;
• resin opal, honey-yellow with a resinous luster;
• wood opal, caused by the replacement of the organic material in wood with the opal;
• menilite brown or grey;
• hyalite, a colorless glass-clear opal sometimes called Muller's Glass;
• geyserite, deposited around hot springs or geysers; and
• diatomite or diatomaceous earth, the accumulations of diatom shells or tests.