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Easy when you know…
You create a new batch by taking a different number of diamonds from each batch. One from the first, two from the second, three from the third…and so on until you take ten from the tenth batch – a total of 55 stones.
You then weigh your new batch. The key is how much lower this reading is than you would have got if you had been weighing genuine diamonds only.
Let’s imagine that the difference between the reading you expected and the reading you got was 4mg. Each fake diamond weighs 1mg less than a genuine stone so your batch of 55 must contain 4 fakes. If you made up your batch of 55 as described above, the fake stones must come from batch 4.
So by knowing the weight difference you know which batch holds the fakes.