Prismatic ice was a mysterious magical substance thought to exhibit intense cold.
The legendary Calimemnon Crystal emanated great cold, so some theorized it was incased in or formed of some form of ice, such as exceptionally cold water ice or frozen air. However, the wildest theory put forward was that it might be composed of prismatic ice.
It is completely unknown what prismatic ice could be. By the tone of the statement "or (the wildest of assumptions) prismatic ice?" in Empires of the Shining Sea, page 192, it may be an entirely new and unknown material. The name suggests a relationship to the prismatic ray, sphere and related spells, or to such creatures as prismatic dragons and prismatic golems, which hail from Elysium, but any connection is entirely speculative. In the real world, prismatic ice is simply an ice crystal that functions as a prism, which refracts light.