Most flux-melt and hydrothermal synthetic emeralds are easily detectable, since they present differences in physical properties.
The common refractive indices of the flux-melt product are 1.561-1.564 or 1.565-close to 0.01 below the lowest figure seen in natural gemstone from Colombia. The specific gravity is only about 265-266instead of the approximate 2.71 of natural Colombian material;. A heavy liquid adjusted to a specific gravity of 2.67 serves to separate flux-melt synthetic from natural, although a rare, heavily flawed natural stone may float with the synthetics, and a flux-melt synthetic with many platinum crystals (or a few very large ones) may sink.
Both flux-melt and hydrothermal synthetic emeralds usually contain natural looking inclusions loosely looking like fingerprints. Flux-melt gemstones have them filled with solid flux while hydrothermal synthetics’ ones contain liquid and gas inclusions, and are likely to show nail head spicules as well.
Synthetic emeralds usually show a red fluorescence under ultraviolet detectable under long wave radiation (366 nanometers). In some cases, the fluorescence is more difficult to be detected, so it is recommendable a test in a dark room, with the stone on a dull black background. Rarely, the most highly saturated green natural emeralds exhibit a weak purplish red fluorescence; this effect does not mask the stone’s transparency as it does in flux-melt synthetics.
Gilson synthetic emerald raising the refractive index from 1560-1563 to 1.571-1.579, it did not change the characteristic wisp-like inclusions. It also added a tell-tale 427 nanometer absorption line in the spectrum. The Biron hydrothermal synthetic emerald is inert to both long- and short-wave ultraviolet. In addition to the nailhead spicules that characterize hydro thermal synthetic emeralds, the Biron is unique among synthetics in that it may contain gold inclusions. They are black in transmitted light, but the gold becomes evident when light is reflected from the inclusions.
Another difference between the Chatham product and natural emeralds is the synthetic’s transparency to shortwave ultraviolet light. Natural emeralds are opaque to wavelengths below about 300 nanometers, while the Chatham transmits shortwave ultraviolet energy.
In Linde (now Regency) synthetic emerald the refractive index and specific gravity ranges are approximately those at the lower limits of natural emerald, but distinctive growth patterns can be seen under magnification, just like the synthetic’s inclusions are characteristic, and, together with its red fluorescence serve to distinguish it from natural emerald.