Many rock crystals and their carvers have shown immense skill in their work treasured by caliphs. The tradition of carving rock crystal in Egypt was masterful, the purest crystals were imported from Arabia and Basra, and the islands of Zanj in East Africa. Nasir-i-Khusraw, Persian poet and philosopher states, "Yemen was a source of pure rock crystal, and that prior to discovering this source, lesser quality rock crystal was imported from the Maghreb and India.”[8] Rock crystals has provided valuable insight into the advancement of traditional crafts that are being presented today.