One teaspoons worth of ashes is required to make most cremation ash jewellery.
There are four main techniques used in the creation of jewellery. Making diamonds from ash is one, but they can also be added to resin, glass and the metal itself.
Making diamonds from cremation ashes, whilst technically possible is sadly being offered by companies that probably don't actually provide the service. Instead they sell you a regular diamond at an inflated price.
So it is very much a case of buyer beware when it comes to diamonds from ashes.
The biggest tell for a company NOT making diamonds from your ashes, is that their website will be super vague as to where their machine is and there will be no pictures/videos of it or the lab. Just please be super cautious.
Personally I do not know of a single company that I can hand on heart vouch for.