Leno explained that there was only one true test to tell whether a diamond is real or not - if the stone is bogus, the chemicals in the synthetic material used to make up the gem will cause it to catch fire.
A nervous Kardashian tentatively handed over the pricey piece of jewellery, laughing, "Wait, since I'm into contracts, will you sign a contract so that if something happens to it you will pay the insurance?"
Leno agreed, replying, "The nice thing is that if it's real, then nothing will happen, but if it's fake then obviously this Mr Humphries has some explaining to do."
The comedian then held a lit match up to the ring - and it became engulfed in flames, prompting the studio audience to gasp as Kardashian forced a smile.
But Leno quickly calmed the reality star's fears by revealing it had all been a big prank and that he had switched the ring for a cheap imitation.