Holystone is what makes up a large majority of the islands in the Aether, it looks like an edited Cobblestone texture. When mined it drops Holystone, which can be used to make tools. These tools have a slight chance to drop an Ambrosium Shard when used, this includes swords. Holystone can also be enchanted with an Enchanter to give Healing Stones.
See also Mossy Holystone.