Once a gem has reached level 15, it can be cut into a star gem with the appropriate research.
It costs 5 diamonds to cut. Once cut it can be further upgraded by sacrificing regular gems. Each level of a star gem is equivalent to 5 levels of the treasure.
Unlike regular gems, star gems can have gem skills that give added benefits to all your generals. These include: Increased combat rating during duels, increased rating during attack, increased rating during defense, chance for a critical regular attack, chance for critical tactic attack, chance for reduced tactic damage taken and chance for instant kill front row during regular attack.
Gem skills can be upgraded by paying 10 diamonds or using free polishes gained during the dice event, up to a maximum of level 5.
All gem skills are additive in their chances and bonuses and each gem can have up to four skills depending on its current gem level for a maximum of 120 levels total across all 6 gems.
First gem skill is automatically unlocked, second skill at level 5 , third skill at level 10.