Minerals, rocks, crystals and gemstones: What’s the difference?

Rocks, crystals and gems can all be made up of minerals.

Minerals can be a single element or compound. They are materials that meet five requirements. They must:

Be solid at room temperature (usually)

Have a definite chemical composition that is the same everywhere the mineral occurs (may vary within a specific limited range)

Have an ordered internal structure (the atoms are arranged in an orderly, repeated geometric pattern)

Some of the most common minerals include quartz, calcite, fluorite, talc and micas.

A crystal forms when a mineral’s geometric groups of elements stack on top of one another, repeating their arrangement to form a three-dimensional object with flat faces. The shape that these building blocks take are different from mineral to mineral.

A rock is a solid object made up of mineral grains, but it can have many different minerals inside. It also may contain organic remains. Rocks don’t have the same repetitive shapes that crystals have.

Rocks are divided into three basic types: igneous, metamorphic and sedimentary.

Gems are minerals or stones that have been cut, ground and polished. A finished gem may look very different from the original mineral it was carved from. Gems are often shaped with different flat faces to catch and scatter light.

Gems can also come from organic sources or be made in a lab.

Of the more than 5,000 identified natural minerals, fewer than 100 are used as gemstones and only 16 have achieved importance. These are beryl, chrysoberyl, corundum, diamond, feldspar, garnet, jade, lazurite, olivine, opal, quartz, spinel, topaz, tourmaline, turquoise and zircon. Some of these minerals provide more than one type of gem; beryl, for example, provides emeralds and aquamarines, while corundum provides rubies and sapphires.

Hardness is a major characteristic of gemstones. The Mohs Hardness Scale is used as a way to help identify minerals. A mineral's hardness is a measure of its relative resistance to scratching, measured by scratching the mineral against another substance of known hardness on the Mohs Hardness Scale. Friedrich Mohs, a German geologist and mineralogist, created the scale in 1822.

Except for diamond, because of its hardness, gemstones are cut and polished in any of three ways.

Some are tumbled, or placed in a cylinder with abrasive grit and water, and the cylinder is rotated. Others are cut en cabochon (with a rounded upper surface and a flat underside) and polished on sandstone wheels. And harder gems are cut with a carborundum saw and then mounted in a holder and pressed against a lathe that revolves quickly. A dental engine can also be used to grind facets.

For most transparent gems, faceted cutting is almost always done. This increases the value of the gem. The four most common faceted forms are the brilliant cut, the step cut, the drop cut and the rose cut.

Birthstones are gemstones associated with each month. The list of traditional or ancient birthstones most well-known in the United States originated in Poland between the 16th and 18th centuries. Wearing one’s birthstone was thought to bring good luck or health.

In 1912, the National Association of Jewelers released a list of modern birthstones. This list differed slightly from the traditional, most notably by including only transparent gems.

To this day, jewelers continue to add options to the modern birthstone chart. For example, spinel, citrine and tanzanite are all recent additions.

In the U.S. today, people draw freely from both lists and pick the stones that appeal to them. Some traditional gems are less expensive (like turquoise) or more readily available (like cultivated pearls) than their modern counterparts, so some traditional stones remain popular.

The combined value of U.S. natural and synthetic gemstone output in 2021

of U.S. natural gemstones in 2021. Arizona led the nation in natural production, and Oregon was second. Synthetic gems were manufactured in five states.

Increase of U.S. synthetic gemstone production from 2020 to 2021

Opals are made up of millions of microscopic silica spheres. The majority of the time, this internal structure is haphazard, resulting in common opals with unremarkable exteriors. However, a small percentage have regions where these silica spheres are uniform in size and form orderly arrays. These are known as precious opals.

While other gemstones may appear to be indistinguishable, no two opals look the same. This is because one of their characteristics called “play of color.”

When light hits a precious opal, it will scatter throughout the gemstone and reflect back with varying intensities — some dimmed and others bright. Some of the light is reflecting off of the top layer of silica spheres, while some reflects off of layers below that.

If viewed from the correct angle, the light reflecting from many layers adds together. If the angle is shifted, the distance the light travels between layers is changed.

Precious opals can flash a number of iridescent colors such as yellow, orange, green, blue, red or purple. Different colors have different wavelengths, which translates to varying distances they have to travel. So, colors roughly correspond to silica sphere sizes. Red light has long wavelengths, so the silica beads must be close to 300 nanometers. Those take a very long time to form, making red the rarest opal color.

The circumstances that lead to the formation of precious opal are so uncommon that they occur only in a handful of places. About 95% come from Australia. Other sources of opal include Mexico, Ethiopia, and the U.S.

Although most gemstones are mineral materials, there are a number of organic materials that are considered to be gemstones. The most common of these are pearl, bone, amber, coral, jet and ivory. These materials are produced by organisms then cut into gems and other ornamental objects.

Pearls are produced by mollusks and other shellfish but are still considered gemstones. In fact, a pearl is the modern birthstone for the month of June. People have used them as gems for thousands of years.

Amber is a fossilized resin that was secreted by ancient plants. It usually has a yellow to orange brown color but can be white, green, blue or even black. It is easily cut and polished into bright, light-weight gems.

Coral is a colonial organism that lives in warm, shallow marine waters and often develops reefs. When fossilized, it is a hard calcium carbonate material that can be cut or carved and polished into gems. The coral is usually white, cream or pink in color. Bright red is the most desired and rarest color.

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