So I first put in a diamond at 700C in my oven when I was doing some casting. For about an hour. All that happened was the diamond became 'burnt'. A typical thing that happens when you solder on a diamond and you overheat it.
Then I put it in my other oven at 900C and nothing changed.
Then I put it in a Platinum crucible and heated the bejeezus out of it. The maximum that a oxy-
gas torch can go and only the did it start to evaporate.
You can see the rounded facets in the second picture. It looks like it was 'flame polished', like you can do with wax or perspex.