In the past, gemologists dealt mostly with a few natural gems, unsophisticated synthetics, or assembled stones, glass, and plastic. Many more convincing synthetics and enhanced gems exist today, as well as a greater variety, with some traditional stones being mined in new localities resulting in a wider range of properties. Visual and tactile properties are useful clues to identification and an introduction to some of these properties is given below. Minerals are divided into two general luster categories, metallic and nonmetallic. Metallic luster, a term used with opaque material, can be dull, like a nail, or bright and shiny, like a polished gold ring.
Submetallic luster is intermediate and found in minerals such as sphalerite.
Nonmetallic luster, used in transparent or translucent material, can be divided into many categories, from adamantine to pearly, and are ultimately related to the gems refractive index. Adamantine is the highest nonmetallic luster category, represented by the brilliant luster of diamond. Subadamantine is next lower on the scale and found in high refractive index gems such as zircon and garnet. Vitreous , or glassy, is the most common luster in minerals, with quartz and tourmaline as gem examples. Subvitreous is next lower on the scale.