Opals

Opals are a new resource in CoD. It is currently unclear how to get any.

Spend 150 Opals under the Heroes Altar Hire with gems section for either or
 * One random rare level three crest
 * A special Legendary Hero Card which will randomly hire any non-shardable hero other than Desert Magus (who can be rolled with gems or gotten with a monthly SLHC)