Crystals as shown in a mining cart
Crystals as shown in the mining icon
Crystals are a minor currency. They are obtained through Mining, after clearing a field (depleting the health of 16 tiles).
Other sources of Crystals:
Each crystal may be sold for 
100,000, having the highest value of all the sell-able currencies. They are also used in Soul Orbs.
It was hinted that Crystals will be also used in Guild Upgrades, a feature to be developed.