Alumina
From HvWiki
Aluminium oxide or Alumina is a chemical compound of aluminium and oxygen with the chemical formula Al2O3. Aluminium oxide is an excellent thermal and electrical insulator. It is extremely hard, 9.0 on the Moh's scale (Diamond is 10), which makes it difficult to work with.
Because it is so strong, and an excellent insulator, Alumina is used in high-vacuum high-voltage applications such as the Fusor.
Corundum is the naturally occurring mineral form of aluminium oxide.
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