Moissanite is a manmade stone that looks like a diamond. Its significance is that it tests as 'diamond' on diamond testers. Therefore, if you have a standard diamond tester, you will also need a Moissanite tester (though it's far simpler to get a 'Multi' tester that tests for both). The method is: test the stone on a diamond tester, if it reads diamond, it is either diamond or Moissanite - you won't know which. Then test the stone on a Moissanite tester to see if it is Moissanite. If it registers diamond on a diamond tester and not Moissanite on a Moissanite tester - then it's diamond.