Is it ensure, insure, or assure?
1797, from Italian travertino "a kind of building stone," from Latin tiburtinus, from Tiburs, adjective from Tibur (modern Tivoli), region in Latium.
A white, tan, or cream-colored form of limestone, often having a fibrous or concentric appearance. Travertine is formed through the rapid precipitation of calcium carbonate, especially at the mouth of a hot spring or in limestone caves, where it forms stalactites and stalagmites. It is similar to, but harder than, tufa.