Many tiara bases are fashioned of wire made from metals ranging from the most common such as pot metal to fine metals such as platinum and gold. The wire is artistically arranged to show off both the design of the tiara and the stones to their best advantage. Many tiaras are raised at the center and taper off at the sides. In less expensive tiaras the stones are glued into place, but costly tiaras  set gems in prongs so there is less likelihood of losing a stone.

Excerpt from: What are Tiaras