The Ice Crystal is an item that can only be equipped on X/3 and on X/4 Ice Towers with more than 1000 pops (current game). It makes them capable of freezing White, Zebra, Two-Faced Bloons and M.O.A.B.'s. It needs to be crafted from Ice Shards first.
The Ice Shards are items that are obtained when an Ice Tower gets to 250, 500 and 1000 pops (current game). They are used to craft Ice Crystals (see above). It's obvious that the tower needs Snap Freeze in order to make these appear.