This is a set of 'difficult' stones that are confused with diamond, not so much because they look like diamond to a jewellery expert, but because they are very hard to measure on machines: YAG, GGG, synthetic sapphire, synthetic spinel, zircon, strontium titanate and cubic zirconia.
Diamond testers are good at distinguishing diamond from non-diamond; gem testers are good at categorising gemstones that are obviously nothing like diamond; the difficulty is knowing when you might have one of the above, which are similar to diamond - the only way to know exactly is, on a tester, to compare the readings of actual stones.
They are clearly labelled, do not mix them up, they cannot be returned!
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What mm size are the stones please?