A gemstone is a precious or semiprecious stone that has a culture and tradition of being used to produce fine goods such as jewelry. The precious gemstones are traditionally diamond, ruby, sapphire, emerald and natural pearls. All other gemstones can be considered semi-precious. The following are common types of gemstone.
A diamond is carbon that has crystallized in conditions of extreme heat and pressure deep in the Earth. Diamonds are the hardest natural material meaning that they are resistant to scratches. However, they can be easily crushed with a hammer and aren't as invincible as people may believe. Diamonds range from virtually worthless to priceless based on quality factors known as carat, cut, color and clarity. This is known as the 4Cs. Most diamonds are a colorless crystal but can also be yellow, brown, blue, green, black, pink, orange, purple, and red. For example, the Hope Diamond is a large blue diamond that is blue due to trace amounts of boron. It glows red in the dark and is surrounded in myths and stories such as the superstition that it is cursed.
A ruby is one of the cardinal gems, meaning that it has been considered as the most precious of gems since antiquity. Rubies are a pink to blood-red color of the mineral corundum. As with diamonds, rubies are valued according the the 4Cs. Historically, a mineral known as red spinel was confused with rubies. Rubies are surrounded in much myth and lore. For example, the Chinese historically buried rubies under prominent buildings for luck.
As with rubies, sapphires are a variety of the mineral corundum with the difference being color. Rubies are red or pink and sapphires are usually blue but include other colors of corundum such as yellow, violet, orange and green. Differences in colors reflect impurities in the stone. For example, the red of rubies is chromium and the blue of sapphires is created by titanium and iron.
Emeralds are a green crystal form of the mineral beryl. They are graded by the 4Cs. In order to be considered an emerald a stone must be medium to dark green with light green equivalents being viewed as green beryl. Emeralds often have inclusions that reduce their toughness such that they can be broken. The most precious emeralds are a dark green that is also translucent.
Natural pearls are formed in certain types of mollusks in response to accidental conditions such as the intrusion of sand. These are quite rare such that pearls were traditionally considered a precious gemstone that could be extremely valuable. Cultured pearls are only semi-precious. As cultured pearls now dominate the market for pearls, it is common to drop pearls from the list of precious gemstones.
Opals are a non-crystalline form of silica, the same material that makes up sand, quartz and a type of glass known as fused quartz. Historically, varieties of opal that are iridescent were considered precious. However, this status is now debatable due to the discovery of large deposits of opal, particularly in Australia. Iridescence is an optical property whereby a substance has a slightly different color from different viewing angles and in different lighting conditions. Opals that aren't naturally iridescent are known as common opal and aren't precious. Precious opals can be so iridescent that they can be considered multicolored.(Opal is so colorful that it tends to look unreal in photos)
Amethyst is a violet variety of quartz. It has the same properties as quartz with the exception of its color that ranges from a light pinkish violet to a deep purple. Amethyst was historically one of the Cardinal gems. However, it is no longer viewed as a precious gem because large deposits have been found, particularly in Brazil. The ancient Greeks believed that Amethyst prevented drunkenness and made wine glasses from it. In the middle ages, European soldiers wore amethyst amulets as it was believed to keep people cool-headed.
Like emeralds, aquamarines are a crystal form of the mineral beryl. The difference is that aquamarines are blue or cyan. The exact color of aquamarine depends on the mine they come from and ranges from nearly clear to dark blue. The color of aquamarine can fade in sunlight and can be darkened with high energy particles such as gamma rays. The ancient Romans believed aquamarine was good luck at sea and that it helps remedy laziness.
Beryl is the mineral that produces emeralds in dark green and aquamarine in blue. Other colors of beryl may be valued as gems but can't be described as emeralds or aquamarine. Some colors of beryl, such as red, are quite rare but still aren't valued like emeralds. The value of gemstones has as much to do with the culture, traditions, perceptions and standards surrounding them as their actual rarity. Some colors of beryl are known by individual names as with emeralds and aquamarine. For example, colorless beryl is known as Goshenite.
Turquoise is an opaque mineral that is a hydrated phosphate of copper and aluminum. Its blue-green color is so distinctive that turquoise has become a well known color name. Turquoise was valued by ancient civilizations such as Ancient Egypt, the Aztecs and the Shang Dynasty of China. In modern times, its value has been degraded by market practices such as dyeing and treatments that make low grade stone appear to be higher grade. This type of practice shakes consumer perceptions and generates an environment of uncertainty.
Topaz is an abundant crystal made from a complex silicate that includes aluminum and fluorine. It is often brown to yellow but can also be orange, blue, red, green or pink. Topaz was known to antiquity and is attached to a large number of old myths. For example, in the middle ages it was believed to enhance mental powers and make the body warmer.
Garnet is a set of different silicate minerals with a crystal structure including pyrope, almandine, spessartine, grossular, uvarovite and andradite. These were commonly used as gems by ancient Rome and throughout the middle ages in Europe. The word garnet comes from the Middle English gernet meaning dark red. As a semi-precious gemstone, red varieties are most common.
Onyx is the parallel banded variety of chalcedony, a form of silica composed of fine intergrowths of quartz and moganite. In order to be considered onyx, parallel bands of color must be present in the stone. Onyx has a silky appearance similar to porcelain. It actually has a crystal structure but this is invisible such that it is considered to be a cryptocrystalline form. Onyx comes in many colors but is best known for varieties with white or black bands.
Lapis lazuli is a deep-blue metamorphic rock that has been valued since prehistorical times for its color. It was used by early civilizations. For example, lapis lazuli is prominently used in the Mask of Tutankhamun, one of the world's most iconic artworks and historical artifacts that dates to 1323 BC. Historically it was common to crush lapis lazuli to make a pigment known as ultramarine that is one of the most expensive pigments ever sold. Due to its price, it was common for European painters of the middle ages to use other blues as an undercoating with a thin layer of ultramarine on top.(Mask of Tutankhamun, dark blues are lapis lazuli gemstones)
Jade refers to two minerals that play a prominent role in various Asian cultures: nephrite and jadeite. Jade is mostly known for its green varieties. Dark green jade with a transparent quality is quite valuable alongside other favored colors such as white. Jadeite is generally more valuable than nephrite. There are only about a dozen known deposits of jadeite. Jade is very commonly treated to change its appearance. However, the market is very well developed such that these treatments are commonly detected and graded accordingly.
Amber is fossilized tree resin that, like pearls, is considered a type of organic gemstone. This has been used for decorative and ceremonial purposes since antiquity with evidence of use by prehistoric peoples. Amber often contains organic inclusions such as leaves, insects, small animals, wood, hair, feathers, crustaceans, bacteria and amoebae. The oldest amber is about 320 million years old. Deposits of amber are fairly common but the vast majority of amber isn't considered gemstone quality. Amber is also of interest to museums and collectors of natural history.
Malachite is an opaque copper carbonate hydroxide mineral. It is valued for its green bands of color. It was historically ground up and used as a green pigment. Malachite was commonly worn as an amulet in the middle ages. It was thought to help with sleep and melancholy and was given to Capricorns who were thought to be particularly prone to sadness.
Alexandrite is a strongly pleochroic variant of a mineral known as chrysoberyl. Its pleochroic property means that it appears to be different colors in different types of light. Specifically, alexandrite appears to be a bluish green in sunlight and a purplish red color in artificial incandescent light. This results from strong absorption of yellow light.
Tourmaline is a crystalline boron silicate mineral that comes in a variety of colors due to the presence of aluminium, iron, magnesium, sodium, lithium or potassium. A large number of boron silicate minerals are considered tourmaline. As a crystal that demonstrates a large number of colors and can have a translucent quality, it is popular as a relatively inexpensive gemstone. Tourmaline is also commonly available as a bicolor mineral such as the watermelon tourmaline pictured here.
Peridot is a magnesium iron silicate. Peridot forms in molten volcanic rock of the upper mantle and is also found in meteorites. As a gem it is often a yellowish green or olive green. Mining of peridot occurred as early as 300 B.C on St. John's Island in the Red Sea. As such, there is some culture and history behind its use as a gemstone.
Moonstone is sodium potassium aluminium silicate that displays a schiller meaning that it has a metallic iridescence that originates from below the surface of the stone. This is typically a milky or blue glow of the stone in response to light. As the name suggests, moonstone was associated with the moon in antiquity with myths such as the idea that moonstone is created by moonbeams. Moonstone has experience several waves of modern popularity as it was fashionable in the 1920s when it was associated with the Art Deco aesthetic. It was also popular in the late 1960s and early 1970s due to the moon landings.
Spinel is a type of crystal mineral that mostly consists of magnesium and aluminum oxides. These come in a variety of colors with blue and red generating the most interest as a semi-precious gemstone. Before, 1783 red spinels were commonly confused with rubies. For example, a large red spinel known as the Black Prince's Ruby is prominently featured at the front of the Imperial State Crown of the United Kingdom. This jewel has an interesting history as it was struck with a battle axe while on the head of Henry V of England. Both Henry and the red spinel survived the battle. To be clear, red spinels aren't rubies and shouldn't be marketed as such.
Carnelian is a brownish-red form of chalcedony, the same mineral as onyx. Carnelian ranges from a bright orange to an almost black red. In all of these variations it is often slightly brownish. Carnelian has been used since the Bronze Age to make jewelry such as beads. It is highly workable and is often shaped and engraved. For example, the Ancient Romans often used it to create official seals for stamping documents.
Quartz is a hard, crystalline mineral composed of silicon and oxygen. Several varieties of quartz are viewed as semi-precious based on impurities that give them a distictive color or optical properties. This includes amethyst, citrine, rose quartz, milky quartz and rutilated quartz. Pure quartz, also known as rock crystal, is clear and may also be viewed as semi-precious.