The Light of the Desert is the world’s largest faceted cerussite gem, weighing 898 carats (179.6 g). It is currently part of the collection of the Royal Ontario Museum (ROM) in Toronto.
Cerussite (also known as lead carbonate or white lead ore) is a mineral consisting of lead carbonate (PbCO3), and is an important ore of lead. The name is from the Latin cerussa, white lead. Cerussite crystallizes in the orthorhombic crystal system and is isomorphous with aragonite. Like aragonite it is very frequently twinned, the compound crystals being pseudo-hexagonal in form.
The raw cerussite was discovered in Tsumeb in northern Namibia. It was then acquired by a gem cutter from Arizona, who cut the raw material into the gem on display. The cutting and transport of this gem is a delicate business as cerussite is extremely fragile and sensitive to temperature changes and vibration. After it was cut in Arizona, the gem was placed in a box, then wrapped in a large woolen scarf and a winter vest, and then hand transported to Toronto for display. Cerussite is too fragile to be set in jewelry.
According to wikipedia