I have never heard that, but perhaps an expert will be along shortly to enlighten us. My wife is allergic to jewelry so I have never paid much attention to it. One of the great blessings of my life.
Wiki: “Although the GE stones and natural diamonds were chemically identical, their physical properties were not the same. The colorless stones produced strong fluorescence and phosphorescence under short-wavelength ultraviolet light, but were inert under long-wave UV. Among natural diamonds, only the rarer blue gems exhibit these properties. Unlike natural diamonds, all the GE stones showed strong yellow fluorescence under X-rays.”
I think the following is related to non-diamonds, but still interesting:
Also: “Diamond’s thermal conductivity is made use of by jewelers and gemologists who may employ an electronic thermal probe to separate diamonds from their imitations. These probes consist of a pair of battery-powered thermistors mounted in a fine copper tip. One thermistor functions as a heating device while the other measures the temperature of the copper tip: if the stone being tested is a diamond, it will conduct the tip’s thermal energy rapidly enough to produce a measurable temperature drop. This test takes about 2–3 seconds.”