How can I see if a diamond is genuine?

My honest answer: You can NOT! You need to be an expert and use specialized equipment to make sure a diamond is real. Do not trust ideas like scratching a beer bottle (it may damage the diamond too), thermal conductivity (Moissanite conducts equally well), looking through, comparing weight, etc.
If the diamond is accompanied by a certificate you will know it is real and if it is treated artificially or not.
Tip:If you can not easily find inclusions under a 10 times magnification you should become suspicious, most likely it is not a real diamond. But be aware of the fact that some gas bubbles in cubic zirconium may appear like inclusions if you do not look carefully.

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For further information see Diamonds Glossary.