Earth tiara are a head slot item used in Runecrafting. In can be created by using a tiara on the Earth rune alter with an Earth Talisman. Doing this grants 33 Runecrafting exp.
The main use of the earth tiara is that it lets you enter the earth rune alter with a left click and since it can be equipped it frees up an inventory space that the earth talisman would normally occupy.