Crimstone is a block type found in The Crimson. Before Hardmode, it is most prominent in the lining of Chasms. It requires at least a Deathbringer Pickaxe or explosives to mine.
Crimstone is the Crimson version of Stone (similar to Ebonstone in The Corruption, or Pearlstone in The Hallow). As with Ebonstone, Crimstone can be converted into ordinary Stone via Purification Powder (Pearlstone requires a Vile Powder instead).
In Hardmode, the spread of Crimson converts ordinary Stone blocks into Crimstone. Crimstone itself also spreads Crimson to susceptible blocks nearby.
Unlike its Corruption counterpart (Ebonstone), Crimstone can't be used to make bricks. Instead, it can be crafted into Flesh Blocks at a Meat Grinder, which can then be crafted into furniture (neither Ebonstone nor Ebonstone Brick can be used to craft furniture).