Ambrosium is a natural ore found in the Aether. It is the equivalent of coal in surface Minecraft, but has more properties. With ambrosium shards, you can make torches to light the way or to enchant and restore items. Also, in shard form, it can be eaten to restore half a heart. Ambrosium can also enchant holystone to create healing stone, which heal 2 hearts.
If you have just arrived into the Aether and you need extra Ambrosium it is reccomended that you mine it with a skyroot pickaxe as you'll get double the Ambrosium. Another way to obtain Ambrosium is to use Holystone tools .