If you buy an uncertified diamond, one of the first tests you may want to do is to check that the stone is real and natural and whether it has been artifically treated in any way. This video shows the process and equipment used by diamond grading laboratories to check each diamond.
It is possible to test the diamond using an electronic diamond tester. To be 100% accurate though, you will need to send the stone to a laboratory where it can be tested in greater detail.
The grading laboratories use a piece of equipment developed by DeBeers which is called Diamond View. This instrument makes visible the growth structure of the diamond. The growth structure is very different between natural diamond and synthetic diamond or other types of stone. In nature, there is a continuous growth and dissolution of the diamond which gives an irregular growth pattern.
This is in contrast to what you see in a synthetic diamond which is growing in a systematic and continuous way. A diamond can also be treated to try to enhance its appearance. For example, a diamond can be heated, in order to improve its apparent colour, or it can be drilled with a laser and backfilled with glass to improve the apparent clarity grade.
To check if that has happened the laboratories use a technique called spectroscopy. This involves shining a light at the diamond at an atomic level and looking for the interaction and the light and diamond. A trained person can assess the reading to see if any ‘enhancements’ have been applied to the diamond.