Eternal Ice

Eternal ice is a type of gemstone that crystallizes from ice that has not melted for at least a century in a cold-aligned elemental node. Even crystals of only a century in age won't melt if removed from the area, but the older they are, the more potent the cold energy contained within them. There are legends of ten-thousand-year crystals.
Eternal ice crystals are ideal for summoning elementals of frost and ice. The older the crystal, the more powerful the elemental which can be summoned.