Besides species and varieties, there are broader categories of minerals called groups. A group consists of minerals from several closely related species. Minerals in a group can vary in crystal structure or chemical composition, but they always have common characteristics in at least one of those categories.
Some gemstone groups are tourmaline, spinel, garnet, and feldspar. Members of the garnet, tourmaline, and spinel groups have essentially the same crystal structure but different elements in their chemical composition. Some gems in the feldspar group have the same chemical composition but slightly different crystal structure. Other feldspars have the same structure but different chemical composition.
Feldspars are among the most common of the earth’s minerals, but not all of them are gems. Some are just plain rocks. You might be familiar with one member of the feldspar group: moonstone. It’s a semitransparent to opaque gem with a soft, billowy light effect. This mysterious gem is covered in more detail in the Essential Colored Stone Reference Guide. Shiny sunstone labradorite is also a member of the feldspar group.
The Essential Colored Stone Reference Guide will give you more detail on individual members of gemstone groups. For example, you’ll find several members of the garnet group—almandine, pyrope, rhodolite, tsavorite, and more—that come in colors ranging from red to orange to various shades of green.
