I've seen people saying it's impossible since you can't autoclave them and implosion compressing them gives industrial diamonds rather than normal diamonds, but I found a way to do it. First you implosion compress them into industrial diamonds, which you can then implosion compress again or laser engrave into flawless diamonds. You can then forge hammer the flawless diamonds back into normal diamonds.
#You can convert diamond dust to vanilla diamonds
primal vigil
sinful ginkgo
you can also make the industrial diamonds into a block and then uncraft it into normal