13th century reliquary cross
Any revival of the venerable art of ecclesiastical ivory carving is hampered by restrictions on the ivory trade, consequences of the exorbitant poaching that has depopulated the African elephant herds. But for a legal alternative, fossil ivory from mastodons, mammoths and long-dead walruses can be used; this is actually cheaper than legal elephant ivory. Most fossil ivory is imported from Siberia.
Some fossil ivory has an extraordinary property, due to minerals that have leached into it over the millennia. When baked, it turns blue. In this form it is known as odontolite, or tooth-stone.
The process for creating odontolite was discovered by Cistercian monks in mediaeval France, who mined mastodon ivory from the Pyrenees. Odontolite can be found in numerous works of liturgical art. This article at the website of the European Synchrotron Radiation Facility provides more information.