A vertical crucible called a "skull" is used to make the gem called "cubic" this
The so-called 'skull' melting technique1 has been used only in the case of cubic 
zirconia, because of the very high melting temperature of the... [DOC] [TLE] What are Synthetic Gemstones, Imitation and Simulants | Gem Rock ... [PAR] Apr 19, 2016 ...