That used to be true, but now, and specifically with diamonds, not so much: https://www.brightonsavoy.com.au/why-does-a-diamond-have-no-....
I've been tempted to try making them at home since about 2007, but for some reason all the different people I've lived with since then have objected to me running a lightly modified microwave oven continuously for a month at a time, with a few holes in the heating cavity so I can install an inverted pyrex bowl and pipes to connect it to a low-pressure methane supply system…