A Lava Crystal was a Modifier both for tools and weapons. If added, the tool(s) is/are granted with the power of a furnace, meaning it will auto-smelt blocks when harvested. However, adding this to a weapon will not give it the same effect. Instead, it will set mobs on fire for 3 seconds.
This modifier can be stacked with Luck (Fortune) and is not compatible with Silky (Silk Touch).