Explore the beautiful and fascinating world of gems. Learn about their properties, classification methods, and what makes them unique from other types of rocks and minerals.
What Makes a Gem a Gem?
When you see the word gem, you probably picture fancy stones set in jewelry or royal crowns. Fine jewelry, crowns, scepters, and other famous jewels, like the Hope Diamond, are all examples of gems. Certainly we know a gem when we see one, but what is it that technically makes a gem a gem?A gem, or gemstone, is a type of material that is capable of being cut and polished for use in jewelry or other ornamental applications. Gems are most commonly made of minerals.
Minerals, as you might already know, are naturally occurring, inorganic, crystalline solids, of definite chemical composition. There are also a few ‘gemstones’ that are actually rocks or fossils, not minerals, but we will address those later.
Types of Gems
There are many different types of gemstones, and they can be grouped based on how they are formed. Like mentioned above, gems can be formed from minerals, organic materials, other inorganic materials, or rocks (mixes of minerals, crystals, and other rocks).
The gem-forming minerals are by far the largest group, with over 130 different minerals being used as gems. Diamonds, sapphires, rubies, emeralds, and almost all of the birthstone gems (except for opal) are formed from minerals.
The next largest group is the organic group, which contains both fossil-based gems such as amber, and organism-formed gems like pearl, mother of pearl (nacre), and ivory. This group contains gems that are most frequently used in ornamental carvings and sculptures, as many are structurally strong (like ivory) or delicate and iridescent (like mother of pearl).
Other Inorganic Gems
There are only two other types of inorganic gems, opal and obsidian.
Obsidian is a black volcanic glass, and forms as a product of volcanic eruptions. It also has a ‘snowflake’ variety that has white, snowflake-like inclusions in it.
Opal is an unusual material, in that it is amorphous form of silica, meaning it can be a solid crystal but sometimes behaves as an almost liquid intermediate phase.
Lastly, the rock gems group consists of lapis lazuli, a blue and sometimes white or gold stone, and unakite, a pink, green and white form of granite.
There is a special class of gemstones known as synthetic gemstones.
These are stones that are not naturally occurring, like minerals, or formed from organisms, like pearls, but instead are formed through human intervention. Synthetic gems are designed to look like naturally occurring gems but without the hefty price tag caused by the rarity of natural stones. The most classic example of a synthetic gem is cubic zirconia.
Properties of Gems
Sometimes our classification of gemstones is based on science, and sometimes on marketing hype.
Precious or Semi-Precious
One of the major properties of gemstones are whether they are classified as precious or semi-precious. This is not a measure of how young or cute the stone looks, but mostly a marketing term designed to make certain stones seem more rare or important than others.
Diamonds, emeralds, rubies and sapphires are all considered to be precious, and thereby (according to the sales people) the most valuable and most desirable. All other gemstones are considered semi-precious.
The actual value of a gemstone is largely determined by its rarity and technical characteristics. These characteristics include certain parts of the definition of a mineral (like chemical formula and crystal shape), as well as color, luster, refractive index (how they interact with light), specific gravity, hardness (on the Mohs hardness scale), or inclusions (like the white ones in snowflake obsidian).
There are numerous other characteristics than can be looked at, but those are among the most common.
The Four Cs
You have also seen marketing literature on the Four Cs – color, cut (how it is shaped), carats (how large it is), and clarity (how clearly and unobstructed light passes through it). This was originally designed for use with diamonds, but any gem can be analyzed using those properties.