Welcome to the family of gemstones.In many cases, gemstone groups are based on physical properties, such as water content or optical phenomena, for example cat’s eye gemstones, rutilated gemstones, star gemstones and color change gemstones. Other gemstone groups may be based on more unusual criteria such as organic gemstones, natural glass gemstones, mineraloid or non-crystalline gemstones, and even gemstones that are just rocks.
Interestingly, there are even gemstone groups that are not actually ‘groups’ at all, but rather just a single species; but because of their significance in the gem and jewelry trade, they are justifiably classed as their own group. Some single species gemstone groups include diamond, peridot, zircon, topaz, turquoise and spinel.
The following is a brief summary of some of the most important gemstone groups today:
Apatite is infrequently used as a gemstone. Transparent stones of clean color have been faceted, and chatoyant specimens have been cabochon-cut. Chatoyant stones are known as cat’s-eye apatite, transparent green stones are known as asparagus stone, and blue stones have been called moroxite.
If crystals of rutile have grown in the crystal of apatite, in the right light the cut stone displays a cat’s-eye effect. Major sources for gem apatite are Brazil, Myanmar, and Mexico. Other sources include Canada, Czech Republic, Germany, India, Madagascar, Mozambique, Norway, South Africa, Spain, Sri Lanka, and the United States.
Well-known varieties of beryl include emerald and aquamarine.
Beryl of various colors is found most commonly in granitic pegmatites, but also occurs in mica schists in the Ural Mountains, and limestone in Colombia. Beryl is often associated with tin and tungsten ore bodies. Beryl is found in Europe in Norway, Austria, Germany, Sweden (especially morganite), Ireland and Russia, as well as Brazil, Colombia, Madagascar, Mozambique, Pakistan, Afghanistan, South Africa, the United States, and Zambia. US beryl locations are in California, Colorado, Connecticut, Georgia, Idaho, Maine, New Hampshire, New Mexico, North Carolina, South Dakota and Utah.
Ordinary chrysoberyl is yellowish-green and transparent to translucent. When the mineral exhibits good pale green to yellow color and is transparent, then it is used as a gemstone. The three main varieties of chrysoberyl are: ordinary yellow-to-green chrysoberyl, cat’s eye or cymophane, and alexandrite. Yellow-green chrysoberyl was referred to as “chrysolite” during the Victorian and Edwardian eras, which caused confusion since that name has also been used for the mineral olivine (“peridot” as a gemstone); that name is no longer used in the gemological nomenclature.
Colors: Various shades of green, emerald-green yellow, blue, brownish to greenish black, may be raspberry-red under incandescent light when chromian; colorless, pale shades of yellow, green, or red in transmitted light.
Chrysoberyl can also grow in the country rocks near to pegmatites, when Be- and Al-rich fluids from the pegmatite react with surrounding minerals. Hence, it can be found in mica schists and in contact with metamorphic deposits of dolomitic marble.
Because it is a hard, dense mineral that is resistant to chemical alteration, it can be weathered out of rocks and deposited in river sands and gravels in alluvial deposits with other gem minerals such as diamond, corundum, topaz, spinel, garnet, and tourmaline. When found in such placers, it will have rounded edges instead of sharp, wedge-shape forms. Much of the chrysoberyl mined in Brazil and Sri Lanka is recovered from placers, as the host rocks have been intensely weathered and eroded.
Corundum is a crystalline form of aluminium oxide typically containing traces of iron, titanium, vanadium and chromium. It is a rock-forming mineral. It is also a naturally transparent material, but can have different colors depending on the presence of transition metal impurities in its crystalline structure.
Corundum has two primary gem varieties: ruby and sapphire. Rubies are red due to the presence of chromium, and sapphires exhibit a range of colors depending on what transition metal is present. A rare type of sapphire, padparadscha sapphire, is pink-orange.
Corundum for abrasives is mined in Sri Lanka, Pakistan, Afghanistan, Russia, Zimbabwe and India. Historically it was mined from deposits associated with dunites in North Carolina, US and from a nepheline syenite in Craigmont, Ontario. Emery-grade corundum is found on the Greek island of Naxos and near Peekskill, New York, US.
Fluorite (also called fluorspar) is the mineral form of calcium fluoride, CaF2. It belongs to the halide minerals. It crystallizes in isometric cubic habit, although octahedral and more complex isometric forms are not uncommon.
Pure fluorite is transparent, both in visible and ultraviolet light, but impurities usually make it a colorful mineral and the stone has ornamental and lapidary uses. Industrially, fluorite is used as a flux for smelting, and in the production of certain glasses and enamels.
All species of garnets possess similar physical properties and crystal forms, but differ in chemical composition. The different species are pyrope, almandine, spessartine, grossular (varieties of which are hessonite or cinnamon-stone and tsavorite), uvarovite and andradite. The garnets make up two solid solution series: pyrope-almandine-spessartine and uvarovite-grossular-andradite.
Garnet species are found in many colours including red, orange, yellow, green, blue, purple, pink, brown, black and colourless, with reddish shades most common.
Because the chemical composition of garnet varies, the atomic bonds in some species are stronger than in others. As a result, this mineral group shows a range of hardness on the Mohs scale of about 6.5 to 7.5. The harder species like almandine are often used for abrasive purposes.
Iolite is a magnesium iron aluminium cyclosilicate. Iron is almost always present and a solid solution exists between Mg-rich cordierite and Fe-rich sekaninaite with a series formula.
Typically occurs in contact or regional metamorphism of pelitic rocks. It is especially common in hornfels produced by contact metamorphism of pelitic rocks.
As the transparent variety iolite, it is often used as a gemstone. The name “iolite” comes from the Greek word for violet. Another old name is dichroite, a Greek word meaning “two-colored rock”, a reference to cordierite’s strong pleochroism. It has also been called “water-sapphire” and “Vikings’ Compass” because of its usefulness in determining the direction of the sun on overcast days, the Vikings having used it for this purpose. This works by determining the direction of polarization of the sky overhead. Light scattered by air molecules is polarized, and the direction of the polarization is at right angles to a line to the sun, even when the sun’s disk itself is obscured by dense fog or lies just below the horizon.
Gem quality iolite varies in color from sapphire blue to blue violet to yellowish gray to light blue as the light angle changes. Iolite is sometimes used as an inexpensive substitute for sapphire.
It is much softer than sapphires and is abundantly found Sri Lanka, Australia (Northern Territory), Brazil, Burma, Canada (Yellowknife area of the Northwest Territories), India, Madagascar, Namibia, Tanzania and the United States (Connecticut).
The largest iolite crystal found weighed more than 24,000 carats, and was discovered in Wyoming, US.
Kornerupine is valued as a gemstone when it is found in translucent green to yellow shades. The emerald green varieties are especially sought after. It forms a solid solution series with prismatine. Strongly pleochroic, it appears green or reddish brown when viewed from different directions. It has a vitreous luster.
It was first described in 1884 for an occurrence in Fiskernæs in southwest Greenland. It was named in honor of the Danish geologist, Andreas Nikolaus Kornerup (1857–1883).Although kornerupine was named in 1884, it was not until 1912 that gem-quality material was found and it remains uncommon to this day.
Deposits are found in Sri Lanka, Burma (Myanmar), Canada (Quebec), Kenya, Madagascar, Tanzania, and South Africa.
Moonstone has been used in jewellery for millennia, including ancient civilizations. The Romans admired moonstone, as they believed it was derived from solidified rays of the Moon.
Both the Romans and Greeks associated moonstone with their lunar deities. In more recent history, moonstone became popular during the Art Nouveau period; French goldsmith René Lalique and many others created a large quantity of jewellery using this stone.
Deposits of moonstone occur in Sri Lanka, Armenia (mainly from Lake Sevan), Australia, the Austrian Alps, Mexico, Madagascar, Myanmar, Norway, Poland, India and the United States.
Peridot is one of the few gemstones that occur in only one color: an olive-green. The intensity and tint of the green, however, depends on the percentage of iron in the crystal structure, so the color of individual peridot gems can vary from yellow, to olive, to brownish-green. In rare cases, peridot may have a medium-dark toned, pure green with no secondary yellow hue or brown mask.
The principal source of peridot olivine today is the San Carlos Apache Indian Reservation in Arizona. It is also mined at another location in Arizona, and in Sri Lanka, Arkansas, Hawaii, Nevada, and New Mexico at Kilbourne Hole, in the US; and in Australia, Brazil, China, Egypt, Kenya, Mexico, Myanmar (Burma), Norway, Pakistan, Saudi Arabia, South Africa and Tanzania.
Quartz is a chemical compound consisting of one part silicon and two parts oxygen. It is silicon dioxide (SiO2). It is the most abundant mineral found at Earth’s surface, and its unique properties make it one of the most useful natural substances.
Quartz exists in two forms, the normal α-quartz and the high-temperature β-quartz, both of which are chiral. The transformation from α-quartz to β-quartz takes place abruptly at 573 °C (846 K; 1,063 °F). Since the transformation is accompanied by a significant change in volume, it can easily induce fracturing of ceramics or rocks passing through this temperature threshold.
The scapolites are a group of rock-forming silicate minerals composed of aluminium, calcium, and sodium silicate with chlorine, carbonate and sulfate.
Spinel has long been found in the gemstone-bearing gravel of Sri Lanka and in limestones of the Badakshan Province in modern-day Afghanistan and Tajikistan; and of Mogok in Myanmar.
Spinel is found as a metamorphic mineral, and also as a primary mineral in rare mafic igneous rocks; in these igneous rocks, the magmas are relatively deficient in alkalis relative to aluminium, and aluminium oxide may form as the mineral corundum or may combine with magnesia to form spinel. This is why spinel and ruby are often found together.
Tanzanite is the blue and violet variety of the mineral zoisite (a calcium aluminium hydroxyl sorosilicate), caused by small amounts of vanadium, belonging to the epidote group. Tanzanite is only found in Tanzania, in a very small mining area (approximately 7 km (4.3 mi) long and 2 km (1.2 mi) wide) near the Mirerani Hills.
Tanzanite forms as a brownish crystal and is trichroic — which means it shows three colors — brown, blue and violet concurrently. Heating — either occurring underground naturally by metamorphic processes, or above ground by man in a furnace removes the brown or burgundy color component to produce a stronger violet-blue color and makes the stone “dichroic” which means it only reflects blue and violet.
Rarely, gem-quality tanzanite will heat to a green primary hue, almost always accompanied by a blue or violet secondary hue. These green tanzanite have some meaningful value in the collector market, but are seldom of interest to commercial buyers.
Topaz is a silicate mineral of aluminium and fluorine with the chemical formula Al2SiO4(F, OH)2. Topaz crystallizes in the orthorhombic system, and its crystals are mostly prismatic terminated by pyramidal and other faces. It is one of the hardest naturally occurring minerals (Mohs hardness of 8) and is the hardest of any silicate mineral.
Topaz in its natural state is a golden brown to yellow, a characteristic which means it is sometimes confused with the less valuable gemstone citrine.
Topaz is commonly associated with silicic igneous rocks of the granite and rhyolite type. It typically crystallizes in granitic pegmatites or in vapor cavities in rhyolite lava flows including those at Topaz Mountain in western Utah and Chivinar in South America.
It can be found with fluorite and cassiterite in various areas including the Ural and Ilmensky Mountains of Russia, in Afghanistan, Sri Lanka, Czech Republic, Germany, Norway, Pakistan, Italy, Sweden, Japan, Brazil, Mexico; Flinders Island, Australia; Nigeria and the United States.
Tourmaline is a crystalline boron silicate mineral compounded with elements such as aluminium, iron, magnesium, sodium, lithium, or potassium. Tourmaline is classified as a semi-precious stone and the gemstone can be found in a wide variety of colors.
Zircon is common in the crust of Earth. It occurs as a common accessory mineral in igneous rocks (as primary crystallization products), in metamorphic rocks and as detrital grains in sedimentary rocks. According to the Madras Tamil Lexicon the name comes from the word “thoramalli” (තෝරමල්ලි) or “tōra- molli”, which is applied to a group of gemstones found in the southern parts of the Indian Subcontinent. According to the same source, the Tamil “tuvara-malli” (துவரைமல்லி) and “toramalli” are also derived from the Sinhalese (Sri Lankan Mother Toungue) root word.
Colors: Reddish brown, yellow, green, blue, gray, colorless; in thin section, colorless to pale brown