Red Ice Block

The Red Ice Block is the Crimson version of Ice, created when Crimson spreads through a normal, blue-ice biome or area.

It may be generated once the Wall of Flesh is defeated for the first time upon entering Hardmode (only on a Crimson world), if the crimson beam originating from the Underworld passes through an underground Snow Biome (This seems to be more frequent on small worlds).

Crimson seems to spread more slowly from Red Ice blocks to regular Ice in comparison to the same situation with Stone or Grass, although this is unconfirmed.

Trivia

 * Possibly references the SCP-009 "Red Ice", from the well-known internet site SCP Foundation.