How can you tell if a stone which is being sold as a diamond really is a diamond? There are several tips you can use as a guide - but, remember, only a trained gemmologist or a good jeweller will be able to tell you for sure. Here are a few things to keep in mind:
1) For loose diamonds - draw a pencil line on a piece of white paper, then place the stone table down on top of the line. If you can still see the line, the stone is nearly certainly not a diamond (or else a really badly cut diamond).
2) 'Huff' your breath on the stone - if the breath mist disappears almost immediately - in fact, before you can even look at it - the stone is nearly certainly a diamond. Diamonds have very high thermal conductivity and so transfer heat very rapidly. If the mist lingers a few seconds, it is not a diamond.
3) A diamond is cold to the touch. Other transparent gemstones, like white sapphire or white topaz, are warm when touched to your lip.
4) The 'heft'. Diamonds are lighter for their size compared to synthetic stones, like moissanite and cubic zirconia, so weigh it on your fingers.
5) Diamonds are singly reflective - that means, when you look at the stone from a sideways angle, you should not see the doubling of facets or inclusions. However, another singly reflective transparent stone is white spinel, so be careful there.
6) Inclusions - spinels tends to be quite clean. CZ and moissanite are extremely clean - you may even see little bubbles in them or mould marks. Diamonds are virtually never clean - look for the black spots, feathers and little cracks which are the hallmark of a natural material. You may need a magnifying glass for this.
7) Moissanite and CZ are extremely sparkly. Too sparkly. Diamonds have a fire and depth that is hard to mimic. Certainly, you will see no 'depth' if you look at the former under magnifying glass.
8) Look at the cut - diamonds are the hardest known natural material. The facets are very sharp and very clean. In a quality stone, the joints are extremely precise. In a glass or paste substitute, the facets can be worn at the edges and the joints, well, disjointed.
9) Look at the setting - an expensive stone will be in a very good setting - 18k gold or platinum, substantial and solid.
10) When you are buying, ask about the clarity and colour. The buyer may not know them. This does not mean the stone is a fake, but it is certainly worth their while - and yours - to obtain guidance from a jeweller.
Finally, buy a diamond tester. You can get one for £30 and it could well save you from making a lot of very expensive fakes. Like that 2ct diamond I bought that turned out not to be a diamond ...