Diamonds

In the game, Diamonds are used for
 * 1) Revive when you died in a stage - 1 Diamond per Revival
 * 2) Recover your stamina - 1 Diamond recover full stamina
 * 3) Expend your Inventory list - 1 Diamond for 5 slots
 * 4) Expend your Friend list - 1 Diamond for 5 slots
 * 5) Diamond Seal Card - 5 Diamonds for 1 Diamond Seal Card

Diamonds can be obtained by
 * 1) Clearing a stage - Clearing Glacial Iceberg, Afire Volcano, Misted Woodland, Holylight Palace, Dark Cove and Lost Relic stages gives you 1 Diamond. Clearing Enochian Tower stages gives you 2 Diamond.
 * 2) Purchase it from the Store
 * 3) Once in a while, Diamonds are given out as a daily event.