Its true they are harder.I'm a collector of rare gems and metals.I work with them everyday. I bought a set of book ends at a yard sale .The man swore up and down it was just a molten down piece of glass. I said ok Ill buy them . I already knew they were diamonds. But to my Superise when I got home, the first thing I did was ,put them to my diamond grinder wheel to get all the conglomerete rock they were encrusted in off .Next thing I knew It ate up my diamond wheel like it was corundum.So I do agree with Serris also they need to go back to the drawing board regarding the thermal diamond tester . No offence guys.