Alphabetical List of Precious and Semiprecious Gemstones

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Precious and Semiprecious Gems

Precious gemstones: Garnet, Imperial Topaz, Ruby, and Sapphire
Precious gemstones: Garnet, Imperial Topaz, Ruby, and Sapphire. Arpad Benedek / Getty Images

A gemstone is a crystalline mineral that can be cut and polished to make jewelry and other ornaments. The ancients Greeks made a distinction between precious and semiprecious gems, which survives to this day. Precious stones were hard, rare, and valuable. The only "precious" gemstones are diamond, ruby, sapphire, and emerald. All other quality stones are called semiprecious, even though they may not be less valuable or beautiful. Today, mineralogists and gemologists describe stones in technical terms, including their chemical composition, Mohs hardness, and crystal structure.

Here is an alphabetical list of important gemstones, with photographs and their key characteristics.

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Agate

Agate is a striped or banded form of the mineral chalcedony.
Agate is a striped or banded form of the mineral chalcedony. Auscape / Getty Images

Agate is crytocrystalline silica, with a chemical formula of SiO2. It is characterized by rhombohedral microcrystals and has a Mohs hardness ranging from 6.5 to 7. Chalcedony is one example of gemstone quality agate. Onyx and banded agate are other examples.

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Alexandrite or Chrysoberyl

Alexandrite gemstone
Alexandrite gemstone. Science Photo Library / Getty Images

Chrysoberyl is a gemstone made of beryllium aluminate. Its chemical formula is BeAl2O4. Chrysoberyl belongs to the orthorhombic crystal system and has a Mohs hardness of 8.5. Alexandrite is a strongly pleochromic form of the gem that can appear green, red, or orange-yellow, depending on how it is viewed in polarized light.

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Amber

Gemstone-quality amber is translucent.
Gemstone-quality amber is translucent. 97 / Getty Images

Although amber is considered a gemstone, it's organic rather than an inorganic mineral. Amber is fossilized tree resin. It's usually golden or brown in color and may contain inclusions of plants or small animals. It is soft, has interesting electrical properties, and is fluorescent. Generally, the chemical formula of amber consists of repeating isoprene (C5H8) units.

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Amethyst

Amethyst gemstone is a purple form of quartz.
Amethyst gemstone is a purple form of quartz. Sun Chan / Getty Images

Amethyst is a purple variety of quartz, which is silica or silicon dioxide, with a chemical formula of SiO2. The violet color comes from irradiation of iron impurities in the matrix. It is moderately hard, with a Mohs scale hardness around 7.

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Apatite

Apatite is a relatively soft blue-green gem.
Apatite is a relatively soft blue-green gem. Richard Leeney / Getty Images

Apatite is a phosphate mineral with the chemical formula Ca5(PO4)3(F,Cl,OH). It's the same mineral that comprises human teeth. The gemstone form of the mineral displays the hexagonal crystal system. Gems may be transparent or green or less commonly other colors. It has a Mohs hardness of 5.

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Diamond

Pure diamond is colorless crystal carbon. It has a high refractive index.
Pure diamond is colorless crystal carbon. It has a high refractive index. De Agostini / A. Rizzi / Getty Images

Diamond is pure carbon in a cubic crystal lattice. Because it's carbon, its chemical formula is simply C (the element symbol of carbon). Its crystal habit is octahedral and it is extremely hard (10 on the Mohs scale). This makes diamond the hardest pure element. Pure diamond is colorless, but impurities produce diamonds that may be blue, brown, or other colors. Impurities may also make diamond fluorescent.

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Emerald

The green gemstone form of beryl is called emerald.
The green gemstone form of beryl is called emerald. Luis Veiga / Getty Images

Emerald is the green gemstone form of the mineral beryl. It has a chemical formula of (Be3Al2(SiO3)6). Emerald displays a hexagonal crystal structure. It is very hard, with a rating of 7.5 to 8 on the Mohs scale.

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Garnet

Grossular var. Hessonite. Garnet comes in several colors and crystal forms.
Grossular var. Hessonite. Garnet comes in several colors and crystal forms. Matteo Chinellato - ChinellatoPhoto / Getty Images

Garnet describes any member of a large class of silicate mineral. Their chemical composition varies, but may be generally described as X3Y2(SiO4)3. The X and Y locations may be occupied by a variety of elements, such as aluminum and calcium. Garnet occurs in almost all colors, but blue is extremely rare. Its crystal structure may be cubic or rhombic dodecahedron, belonging to the isometric crystal system. Garnet ranges from 6.5 to 7.5 on the Mohs scale of hardness. Examples of different types of garnets include pyrope, almandine, spessartine, hessonite, tsavorite, uvarovite, and andradite.

Garnets are not traditionally considered precious gems, yet a tsavorite garnet may be even more expensive than a good emerald!

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Opal

Opal is a soft silicate gemstone.
Opal is a soft silicate gemstone. aleskramer / Getty Images

Opal is hydrated amorphous silica, with the chemical formula (SiO2·nH2O). It may contain from 3% to 21% water by weight. Opal is classified as a mineraloid rather than mineral. The internal structure causes the gemstone to diffract light, potentially producing a rainbow of colors. Opal is softer than crystal silica, with a hardness around 5.5 to 6. Opal is amorphous, so it does not have a crystal structure.

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Pearl

Pearl is an organic gemstone produced by a mollusk.
Pearl is an organic gemstone produced by a mollusk. David Sutherland / Getty Images

Like amber, pearl is an organic material and not a mineral. Pearl is produced by the tissue of a mollusk. Chemically, it is calcium carbonate, CaCO3. It is soft, with a hardness around 2.5 to 4.5 on the Mohs scale. Some types of pearls display fluorescence when exposed to ultraviolet light, but many do not.

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Peridot

Peridot is a green gemstone.
Peridot is a green gemstone. Harry Taylor / Getty Images

Peridot is the name given to gem-quality olivine, which has the chemical formula (Mg, Fe)2SiO4. This green silicate mineral gets it color from magnesium. While most gems occur in different colors, peridot is only found in shades of green. It has a Mohs hardness around 6.5 to 7 and belongs to the orthorhombic crystal system.

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Quartz

Rare rose quartz crystals.
Rare rose quartz crystals. Gary Ombler / Getty Images

Quartz is a silicate mineral with the repeating chemical formula SiO2. It may be found in either the trigonal or hexagonal crystal system. Colors range from colorless to black. Its Mohs hardness is around 7. Translucent gemstone-quality quartz may be named by its color, which it owes to various element impurities. Common forms of quartz gemstone include rose quartz (pink), amethyst (purple), and citrine (golden. Pure quartz is also known as rock crystal.

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Ruby

Ruby is the red gemstone form of the mineral corundum.
Ruby is the red gemstone form of the mineral corundum. Harry Taylor / Getty Images

Pink to red gemstone-quality corundum is called ruby. Its chemical formula is Al2O3:Cr. The chromium gives ruby its color. Ruby exhibits a trigonal crystal system and a Mohs hardness of 9.

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Sapphire

Sapphire is any gemstone-quality corundum that is not red.
Sapphire is any gemstone-quality corundum that is not red. Harry Taylor / Getty Images

Sapphire is any gem quality specimen of the aluminum oxide mineral corundum which is not red. While sapphires are often blue, they can be colorless to any other color. Colors are due to trace amounts of iron, copper, titanium, chromium, or magnesium. The chemical formula of sapphire is (α-Al2O3). Its crystal system is trigonal. Corundum is hard, around 9.0 on the Mohs scale.

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Topaz

Topaz is a silicate gemstone that occurs in many colors.
Topaz is a silicate gemstone that occurs in many colors. De Agostini / A. Rizzi / Getty Images

Topaz is a silicate mineral with the chemical formula Al2SiO4(F,OH)2. It belongs to the orthorhomic crystal system and has a Mohs hardness of 8. Topaz may be colorless or nearly any color, depending on impurities.

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Tourmaline

Tourmaline comes in various colors. A single crystal may contain multiple colors.
Tourmaline comes in various colors. A single crystal may contain multiple colors. Sun Chan / Getty Images

Tourmaline is a boron silicate gemstone that may contain any of a number of other elements, giving it a chemical formula of (Ca,K,Na,[])(Al,Fe,Li,Mg,Mn)3(Al,Cr, Fe,V)6
(BO3)3(Si,Al,B)6O18(OH,F)4. It forms trigonal crystals and has a hardness of 7 to 7.5. Tourmaline is often black, but may be colorless, red, green, bi-colored, tri-colored, or other colors.

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Turquoise

Turquoise is an opaque gem, often seen in shades of blue, green, and yellow.
Turquoise is an opaque gem, often seen in shades of blue, green, and yellow. Linda Burgess / Getty Images

Like pearl, turquoise is an opaque gemstone. It is blue to green (sometimes yellow) mineral consisting of hydrated copper and aluminum phosphate. Its chemical formula is CuAl6(PO4)4(OH)8·4H2O. Turquoise belongs to the triclinic crystal system and is a relatively soft gem, with a Mohs hardness of 5 to 6.

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Zircon

Zircon comes in a wide range of colors.
Zircon comes in a wide range of colors. Richard Leeney / Getty Images

Zircon is a zirconium silicate gemstone, with the chemical formula of (ZrSiO4). It exhibits the tetragonal crystal system and has a Mohs hardness of 7.5. Zircon may be colorless or any color, depending on the presence of impurities.