You can also add the star corundums to that list.
I believed this myself for quite a while.
I got a job to set some natural sapphire crystals into some stuff
one time. I decided to warm them, drop them onto some molten wax and
let them cool to form the seats. With just a little heat, the first
one broke in two. I was stunned. I was shocked. I was amazed. Nothing
had ever happened quite like this. I had been real gentle about the
heat. I let the boss know what had happened. He was alot more
forgiving than the crystal. On the second crystal, I followed the
same procedure. Wham! It happened again. What I believed was a
million to one occurance happened twice in a row.
Later in a GIA course, I learned that corundum can have two phase
inclusions. When discovered in a stone it indicates that that stone
has never been cooked. Good idea not to heat untreated sapphires. If
you do choose to heat stones larger than mellee, make sure that you
know it has previously been heat treated…
Bruce D. Holmgrain
JA Certified Master Bench Jeweler