It’s easy confusing gems, minerals, and rocks! Here is a look at the difference between them.
A gem or gemstone is a valuable cut and polished solid that finds use in jewelry and other adornments. Usually, gems come from mineral crystals. A mineral, in turn, is a natural solid that has a crystalline structure and well-defined chemical composition in pure form. Rocks consist of one or more minerals.
Gem vs Mineral vs Rock
- A gem is a valuable ornamental form of a mineral, rock, or other solid.
- A mineral is a natural crystalline solid with a well-defined composition.
- A rock consists of one or more minerals.
A Closer Look at Gems
Gems are beautiful and often rare, making them valuable.
Most gems come from mineral crystals. For example, rubies and sapphires are cut and polished corundum crystals. Corundum is a mineral with the chemical formula Al2O3. Rubies are red because chromium displaces some of the aluminum in the chemical formula (Al2O3:Cr). Sapphires are corundum crystals in any color besides red, consisting of either pure aluminum oxide or else traces of iron, vanadium, chromium, titanium, or magnesium. But, color alone isn’t always enough to turn a mineral into a gem. The crystal needs to be “gemmy” or translucent.
Under the strictest definition, minerals have geological origin. However, some gemstones are organic. Examples are pearl and amber. Both pearl and amber are fairly soft, but they have value in jewelry because of their rarity and appearance.
Other gems are synthetic. In some cases, synthetic stones are man-made versions of natural stones. Examples include synthetic emerald and sapphire. These gems are more or less chemically identical to natural stones. Other gems are almost exclusively made in the lab, such as moissanite and cubic zirconia.
Some gems are polished rocks. Rocks contain mineral aggregates. An example of an esthetically pleasing rock that gets cut as a gem is lapis lazuli. Lapis lazurite mainly consists of the blue mineral lazurite, plus it contains calcite (white), pyrite (golden metallic), sodalite (blue), and sometimes additional minerals.
Examples of Minerals and Their Gems
A single mineral often gives rise to multiple gems, depending on its exact chemical composition and the conditions of its crystallization. Other gems are known by their mineral names. Here are some examples of minerals and their gems:
- Agate (rock mostly made of quartz and chalcedony): Agate
- Beryl (Be3Al2Si6O18): Aquamarine, morganite, emerald
- Chrysoberyl (BeAl2O4): Alexandrite, catseye
- Corundum (Al2O3): Ruby, sapphire
- Diamond (C): Diamond
- Jade (either nephrite or jadeite): Jade
- Olivine [(Mg,Fe)2SiO4]: Peridot
- Opal (SiO2ยทnH2O): Opal
- Quartz (SiO2): Amethyst, citrine, rock crystal, rose quartz
- Spinel (MgAl2O4): Spinel
- Topaz Al2SiO4(F,OH)2]: Topaz
Precious vs Semi-Precious Gems
Traditionally, the only four precious stones are diamonds, emeralds, rubies, and sapphires. They are “precious” because of their rarity, quality, durability and cost. Semi-precious stones typically have a hardness of 7.5 or lower and are often more abundant and less expensive. Semi-precious gemstones include pearls, opals, jade, amethyst, etc.
But, some so-called semi-precious gems are rarer than precious gems. For example, tanzanite is rarer than diamond. Also, some semi-precious gems are more expensive than precious gems. For example, black opal and jadeite often cost more, per carat, than diamonds.
Not all Minerals Make Gems
Some minerals are not suitable for making gemstones. Some are too soft or fragile. Others are toxic. Still others just aren’t considered attractive.
For example, the mineral mica often has a shimmery, pearlescent appearance. But, it’s very soft and flakes into sheets, so it’s not appropriate for use as a gem. Another example is monazite. Monazite comes in a variety of colors, including orange, purple, pink, blue, and green. It has a Mohs hardness between 5.0 and 5.5 and forms beautiful crystals. However, you won’t see it as a gem because it contains thorium and uranium, so it’s radioactive.
References
- Busbey, A.B.; Coenraads, R.E.; Roots, D.; Willis, P. (2007). Rocks and Fossils. San Francisco: Fog City Press. ISBN 978-1-74089-632-0.
- Dyar, M. D.; Gunter, M. E.; Tasa, D. (2007). Mineralogy and Optical Mineralogy. Mineralogical Society of America. ISBN 978-0-939950-81-2.
- Rafferty, John P., ed. (2011). Geology: Landforms, Minerals, and Rocks. Rosen Publishing Group. ISBN 978-1615304899.
- Wise, R. W. (2006). Secrets of the Gem Trade: The Connoisseur’s Guide to Precious Gemstones. Brunswick House Press. ISBN 0-9728223-8-0.