Touching the crystal, natural crystal is usually cooler than man-made crystal on temperature.
Observed with the eye, natural crystalline inclusions usually have cotton wool, this is a man-made crystal does not have.
For such a color citrine amethyst crystal, usually two to observe its color, even in the top of the amethyst is also a color of yellow crystals, can be identified by this method is additive.
These are the most simple way, simple to understand.