Gems collected power up your crabs. Completing sets of gems will provide further damage boost.
Gems are obtained from Wildebeasts every 4th and 7th day. Each gem increases crab damage by 10% each depending on a specific color collected (i.e. Amethyst, Emerald or Garnet). Regardless of damage increase, the same gem can be collected again.
A single set for each crab type has 10 gems, by completing a set of Gem the specified crab type gets an extended damage boost.
Collecting a complete set of each color will add an additional multiplier boost to your overall damage.