You will never know how accurate a gem tester is (any gem tester) unless you compare the readings with actual stones.
Sometime this isn't so important, for instance if you are 90% certain a stone is paste (glass) and want to 'prove' to a customer that it's not sapphire.
Sometimes it's critical, for instance when you have a stone that does look like diamond, but you don't want to say to your customer, "It's something that looks like diamond but isn't, and I've no idea what it is, in fact I'm clue-less " You really do need an accurate reading to tell the customer what it is!
Although sold for use with the Presidium Duo tester, they can are useful for using as reference when using virtually any tester, even a jewellers loupe.