Science, Tech, Math › Science Alphabetical List of Precious and Semiprecious Gemstones Share Flipboard Email Print Science Geology Types Of Rocks Landforms and Geologic Features Geologic Processes Plate Tectonics Chemistry Biology Physics Astronomy Weather & Climate By Anne Marie Helmenstine, Ph.D. Chemistry Expert Ph.D., Biomedical Sciences, University of Tennessee at Knoxville B.A., Physics and Mathematics, Hastings College Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels. our editorial process Facebook Facebook Twitter Twitter Anne Marie Helmenstine, Ph.D. Updated November 26, 2019 A gemstone is a crystalline mineral that can be cut and polished to make jewelry and other ornaments. The ancient Greeks made a distinction between precious and semiprecious gems, which is still used. 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 any less valuable or beautiful. Today, mineralogists and gemologists describe stones in technical terms, including their chemical composition, Mohs hardness, and crystal structure. Agate Darrell Gulin / Getty Images Agate is cryptocrystalline 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. Alexandrite or Chrysoberyl Coldmoon_photo / 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 pleochroic form of the gem that can appear green, red, or orange-yellow, depending on how it is viewed in polarized light. Amber Siegfried Layda / Getty Images Although amber is considered a gemstone, it's an organic mineral rather than an inorganic one. Amber is fossilized tree resin. It's usually golden or brown 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. Amethyst Tomekbudujedomek / 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 the irradiation of iron impurities in the matrix. It is moderately hard, with a Mohs scale hardness of around 7. Apatite jonnysek / 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. Diamond Koichi Yajima / EyeEm / 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. 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. Garnet Matteo Chinellato / 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 a 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. Opal aleskramer / Getty Images Opal is hydrated amorphous silica, with the chemical formula (SiO2·nH2O). It may contain anywhere from 3% to 21% water by weight. Opal is classified as a mineraloid rather than a 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 of around 5.5 to 6. Opal is amorphous, so it does not have a crystal structure. Pearl David Sutherland / Getty Images Like amber, a 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 of 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. Peridot Willscape / Getty Images Peridot is the name given to gem-quality olivine, which has the chemical formula (Mg, Fe)2SiO4. This green silicate mineral gets its color from magnesium. While most gems occur in different colors, peridot is found only in shades of green. It has a Mohs hardness of around 6.5 to 7 and belongs to the orthorhombic crystal system. Quartz Anton Eine / EyeEm / 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. Ruby Walter Geiersperger / Getty Images Pink to red gemstone-quality corundum is called ruby. Its chemical formula is Al2O3Cr. The chromium gives ruby its color. Ruby exhibits a trigonal crystal system and a Mohs hardness of 9. Sapphire John Carnemolla / Corbis / VCG / Getty Images Sapphire is any gem-quality specimen of the aluminum oxide mineral corundum that is not red. While sapphires are often blue, they can be colorless or any other color. Colors are created by 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 on the Mohs scale. Topaz Fred_Pinheiro / Getty Images Topaz is a silicate mineral with the chemical formula Al2SiO4(F,OH)2. It belongs to the orthorhombic crystal system and has a Mohs hardness of 8. Topaz may be colorless or nearly any color, depending on impurities. Tourmaline Walter Geiersperger / 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(BO 3)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. Turquoise JannHuizenga / Getty Images Like a pearl, turquoise is an opaque gemstone. It is a 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. Zircon Reimphoto / 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.