0
a gem, a gem stone and a sparkly rock. (?)
Gem
Another word for diamond that starts with an R is Rhinestone.
Jewel, precious stone, nugget and gemstone.
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