Diamonds

Diamonds are rarer than gold.

You can:

Buy gold with diamonds

Refill energy

Buy gembits (in case of you do not have enough)

So, in short, do every thing!

How to get them:

Washed-up chests give a 50% chance of giving Diamonds.

Buy them with real money

Finish team goals(mostly participate on the thing that is your expert thing you can do)