Enchanters are crafted using 8 holystone in a box around a zanite gemstone.
More ambrosium is required for items that are stronger or more heavily damaged. For instance, the skyroot pickaxe in the image on the left is nearly finished while the flame is still at about 70%, but a zanite chestplate with low durability requires about 20 ambrosium shards, which also means a very long wait period.
If fuel runs out, enchanting progress stays until the tool is taken out or more fuel is added. Also enchanter can enchant holystone and make it into healing stones.
1.0.0 Aether mod changes the enchanters name to altar.