Scriber / scratch-test for diamonds

Summary

Scriber, 150mm long, carbide tip. 

Detail

For testing 'rough' (uncut) diamonds -a good simple method,  because most (not all) gemstones can be scratched with a carbide tip whereas diamond cannot.  

For testing CUT diamonds: draw the tip across customer's precious gemstone, if it scratches it, it's not diamond (then the customer sues you for damaging their precious gemstone). Remarkably, customers do buy these as a 'cheap' method of testing cut diamonds, they say, "It's OK, I'll be careful!". 

Is it easy to use?

Yes.

Limitations

Stones (including diamonds) are not necessarily the same hardness in all directions. Diamonds (although hard) are also brittle, it is possible that you will chip or crack the stone.

Once scratched, a polished gemstone is damaged and probably becomes worthless.