When I explain things more than once, I keep trying to make the explanation simpler and clearer.
A mineral is defined by its (essential element) chemistry and crystal structure. There are many sites that explain this.
A gem is defined by its properties. If the gem is also a mineral, these properties are based on its essential element chemistry and crystal structure, but can also be due to:
- Trace elements (Cr and V in emeralds, Cr in rubies)
- Inclusions (cat's-eyes and star stones)
- Texture (the interlocking texture in jadeite, the felted texture in nephrite)
- Grain size and lack of inclusions, leading to diaphaneity: transparent to translucent to opaque; the greater the transparency, the greater the likelihood a sample is gem-quality.
As a related topic, I see the term crystal used to mean different things to different people in the gem and mineral business.
- Mineralogically, a single crystal is a connected structure retaining orientation from one end to the other. (In a mosaic crystal, sub-blocks of the structure have slightly different orientations, which can lead to large changes on orientation from one end to the other, like in saddle-shaped dolomite crystals.)
- To an emerald dealer, "crystal" means a fine quality emerald that will hold up well to cutting.
- To a mineral collector, a crystal should retain all its natural surfaces; but to someone in the metaphysical community, polished surfaces may be acceptable.
- By people who deal in glass imitations, I have seen "crystal" used as it is used for glassware: a high-refractive index material (often containing lead) that would still be considered amorphous to mineralogists.
- A crystal ball should be quartz, not glass; sometimes "crystal" is short-hand for rock crystal quartz, the colorless transparent variety.