Normally anything which has a specific gravity (density) greater than water - and diamonds have a density of about 3 ( as someone else said) doesn’t float. BUT I used to get Boy Scouts to rub a needle gently over a candle, wipe it with their scarf(!!) and place it gently on a saucer of water, when they saw that although it was steel, it DID float. If they magnetised it first by stroking it with a magnet it became an El Cheepo compass. SO: you can do that with diamonds too - if you really want. But I insisted that they tell me WHY it floated!
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