The ability of a mineral to let light through. Broken down into 3 broad categories.
Transparent – Mineral lets most of the light through. Objects can be seen clearly though the specimen. For most gemstones, the more transparent, the rarer and more expensive. examples – Clear Quartz (rock crystal), some Calcite, cut Diamonds
Translucent – Mineral lets some, but not all light through. Objects may be seen, but not clearly. This has a broad range, from specimens that are nearly transparent (with some clouding or impurities), to specimens that appear opaque until a bright light is used and some light might be seen around the edges. examples – Agate’s, Selenite, Calcite
Opaque – Mineral let’s no light through. examples – Most metals, Jasper, Unakite, Howlite
Some minerals can be found in all the categories, so translucent to opaque, transparent to translucent, and even transparent to opaque are possible.