A naturally occurring mineral that has been polished and cut to shape to be used for decoration. It may be classed as precious, e.g. diamond, sapphire, emerald, or semi-precious, e.g. garnet, topaz, zircon.
Mike Allaby
Chemistry
Designating molecules in which two functional groups are attached to the same atom in a molecule. For example, 1,1-dichloroethane (CH3CHCl2) is a gem dihalide and can be named gem-dichloroethane. Comparevicinal.