Species, Varieties, and Mineral Groups

Gemologists divide natural gem minerals into gem species. A gem species is a broad gem category based on chemical composition and crystal structure. 

The mineral species beryl, for example, is made up of a regular, repeating structure of beryllium, aluminum, oxygen, and silicon atoms. Those atoms—in the proper arrangement and relative quantities—always define that mineral species. But the broad species name isn’t enough to describe the wealth of color—glorious greens, blues, pinks, reds, and yellows—of a gem species like beryl.

Gemologists use the term variety to describe these color variations. Variety is a subcategory of species, based on color, transparency, or phenomenon (a phenomenon is a special optical effect, like the star in star sapphire, or the blue sheen in moonstone). In the beryl species, for example, aquamarine is the most widely known blue variety, and emerald is the highly prized green variety. 

Sometimes several gem species differ only slightly from each other. Because they’re so similar, gemologists might classify them as a group. A group is a family of closely related mineral species that share the same crystal structure and basic chemistry, but differ slightly in other details. These differences often lead to differences in physical and optical properties. There are many mineral groups, but only a few contain important gem species. The best-known gem groups are garnet, feldspar, and tourmaline.

The important species in the garnet group—almandine, pyrope, spessartine, grossular, and andradite—share the same crystal structure and basic chemistry, but they have slight differences in chemical composition. For example, almandine is rich in iron, while pyrope contains significant amounts of magnesium. You’ll learn more about this in Assignment 24.

A gem’s rarity can be an important part of its value, and that rarity is very closely related to the way it forms within the earth. For example, the formation conditions that produce red garnets are far more common than those that produce ruby. Another factor is that some gems contain more widely available elements than others, so they’re more prevalent. You’ll learn more about how gems form in Assignment 2.

A gem’s rarity and beauty affect how it’s marketed. These factors also affect its value. They are just as important to understand as things like crystal structure and chemical composition.