Sapphire

Sapphire gems are characterized by their ability to inflict 20% slowing debuff on enemy units, above-average range, blue damage type and low single-target damage. Special combinations they're part of always have some sort of slowing effect.

Waves that are weak to Sapphire are 10,19, 27 and 35. On Survival mode, you can begin with Silver, which does blue damage.