A gem generally refers to a gemstone, a cut rock or mineral. A gemstone (also called a jewel) is a piece of mineral crystal, which, in cut and polished form, is used to make jewelry or other adornments.

Source here.

See also:

• crystal
• gem
• amethyst_(gem) (purple)
• aquamarine_(gem) (light blue)
• diamond_(gem) (white)
• emerald_(gem) (green)
• jade_(gem) (light green)
• onyx_(gem) (black)
• opal_(gem) (multicolored)
• pearl_(gem) (shiny white)
• quartz_(gem) (white)
• pink_quartz_(gem) (pink)
• ruby_(gem) (deep red)
• sapphire_(gem) (deep blue)
• topaz_(gem) (mostly brown)
• jewelry

• gem_(species) - popularized by the series Steven Universe