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Gem's.
Gem's could also be 'gem is' so you have to note the context of the word to tell if it is possessive or 'is'
eg
The gem's green -- gem's = gem is
The gem's colour is green. -- gem's = possessive
october gem is a gifted gem from the undead
gem is son gem is sen
gem is send-d
gem i son
there is no gem
gem of wisdom