How can I tell the difference between a sapphire or other stone and a diamond?

I have a necklace with a yellow colored rectangular "diamond" in it. I would say it is a pretty large stone. Upon closer inspection, it seems there is a crack or some sort of abrasion on it. Diamonds are extremely hard and super resistant to such things, so therefore I think it's safe to assume that this is not a diamond, right? If it's not a diamond, what could it be? A Sapphire? And what is the difference in value between the two?|||If you take it to a jeweler, ask them to carbon test it. They have these little things that look like a pen, and when it is touched to a genuine diamond, it buzzes. Diamonds are more expensive than sapphires. But Sapphires are blue. If it IS real, I'd suggest you get it appraised by three different jewelers, and have it insured.|||Sapphires come in all colors of the rainbow - just about. Post below is incorrect.

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|||Poor quality diamonds often are of poor color, such as yellow, and have what look like cracks in them. The cracks are called faults. Diamonds can also have little bits of crud in them, referred to as inclusions.





The more faults, inclusions, and color in a diamond, the lower its quality and value.





On the Mohs scale, the scale that determines hardness of gemstones, sapphires are a 9, while diamonds are a 10, making a good quality sapphire about as unlikely to crack as a good quality diamond.





The answerer above is wrong, btw. Sapphires come not only in blue, but in pink, yellow, clear, purple and green.|||It could be citrine. Most common sapphire are in blue and there are also colourless sapphire or you could be having aYellow sapphire, If its citrine, the value are quite low but if its a yellow sapphire, it would depend on the cutting and inclusion. Best way is to brign them to a jewelry shop and have them look at it. Best of luck.|||Most likely what you have is really a CZ. Diamonds do not crack, they are the hardest substance known to man and sapphire is the next hardest. So a real diamond even if poor clarity would not crack. That is why jewelers use a diamond to cut a diamond because nothing else can.