Black diamond ?

[Mark Armstrong]

Hi. I've recently taken a interest in gems and have just purchased what was listed as a black diamond ring set in 9ct gold from a local auction in the UK. After blindly buying it tried to figure out how to determine whether or not it's a real , heat or uranium treated stone. I realise that a diamond tester is useless but had it tested anyway, negative result. I'm quite sure that I'm not lucky enough to have a real gem but now trying to find out who the maker was ?
Stamped MJ on the hallmarks . Any information most welcome.