Skull crucible
From The Gemology Project
The "skull-crucible" of "skull-melting" method is mainly used to create synthetic cubic zirconia (atleast when it comes to synthesizing gemstones).
The "skull" part refers to the thin crust (or skull) of white zirconium dioxide that is formed around the crystallized core.
Although cubic zirconia (ZrO2) occurs naturally in nature, it is mainly monoclinic of nature and is known as baddeleyite. The cubic form, known as "arkelite" occurs sometimes in zircon.
