Gems are nearly as traditional slot symbols as fruits, with hundreds of slots centered around this theme. This is because gems are valuable, and you can win valuable prizes while playing these slots, too.
As mentioned above, jewels and gems are very often featured as symbols in classic 3-reeled slots, so whenever you play these retro style machines you might find gleaming gems there. video slots, however, often feature gems, too, including some slots based entirely on this theme.
In both classic and video slots the most popular gem symbols are diamonds, as they are the most valuable ones, while rubies and emeralds appear quite often, too. Diamonds may not land on the reels so much often, but they are nearly always very high paying and are also likely to act as wilds. Bonuses in gem themed slots often include some additional features, including extra wilds and multipliers.