corundum, inclusions.5, gems

One of the characteristics that proves that a sapphire has not been heat treated is the presence of coarse, well-formed, undisturbed needle-like inclusions and/or their shorter wedge-shaped relatives. Such inclusions often light up with spectral colors when illuminated by a strong overhead or fiber-optic light source, as seen in the approximately 40-ct sapphire shown in figure 15, which was submitted to the New York laboratory for identification. These needles produce asterism when the stone is cut en cabochon. An interesting trick is to place a small drop of methylene iodide (the 3.32 specific gravity liquid) on the surface of a faceted stone and observe the star effect in the bead of liquid.