Look for color zoning in synthetic diamond which is not present in natural diamond.
Identifying synthetic diamond takes close examination. The most obvious characteristics are the metallic, opaque inclusions that look out of place when compared to natural inclusions. The synthetics appear smooth and reflective under light, due to the metallic catalyst solution used in the growth process. Since the inclusion contains a ferrous metal, it is somewhat magnetic. (Use a strong magnet to test the attraction.) In the case of the type IIb blue synthetics, the boron impurity introduces an electrical conductivity. In the blue synthetics, clouds have been noted that may appear to be similar to cloud occurrences in naturals. The blues may also exhibit color zoning, which does not occur in natural diamond. This zoning is a sudden shift of pattern within the stone, in contrast to a consistent color intensity throughout a natural stone.
In addition: Shortwave ultraviolet light may display a greenish fluorescence in the synthetics; Typically, they will fluoresce stronger under shortwave UV versus long-wave UV; Watch for hourglass or cross-shaped patterns within the stone under normal grading light as well as shortwave ultraviolet.
Using a spectroscope, most natural diamonds will show a 415 nanometer line whereas the synthetic yellows will show a 500-700 nanometer line and the synthetic reds and pinks will show 638.