"Fusion" is applied when two or more characters or items are fused in one, which combine traits of the involveds; for gems (species) fusion is somewhat different.
Gems have the natural ability to fuse, and when it happens they form a being which own not only traits of the gems who originated it, but also some new; these fusions also have multiple limbs and/or eyes (unfused gems don't have these features).
e.g.
Unfused gems
post #1042542 post #982046
Resulting fusion
post #702025
*Posted in general by mistake
Updated by Siral Exan