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The mechanism of asterism effect is similar to chatoyancy, but in this case more than one band of reflections is produced inside a gem, going in different directions, so as a result a moving star can be observed. That means that parallel elongated inclusions must by oriented in at least two different directions.
Such inclusions are commonly produced by the exsolution process, explained in the Epigenetic inclusions section. Exsolution is a very suitable mechanism to generate lots of tiny inclusions distributed in whole the volume of a gem and oriented accordingly to certain crystallographic directions, depending on the crystal structure of the host mineral.
Common examples of gems with asterism are ruby and sapphire with six-pointed stars caused by tiny exsolution rutile needles, or black diopside with four-pointed stars produced by exsolution inclusions of black magnetite. Rose quartz, garnets, moonstone and other gems can also have asterism effect.
Natural diopside with four-pointed star effect. 60.91 ct stone, 23 mm diameter.
Natural high quality star rubies and sapphires are very rare and valuable. However, a very similar effect can be induced in natural stones by treatment consistent in titanium diffusion and slow cooling to provoke exsolution of artificially induced rutile needles. Also, synthetic star ruby and sapphire can be produced by flame fusion method, introducing an excess of titanium oxide and using slow cooling in the same way as for diffused stones.
Same as for chatoyancy, gems with asterism are usually cut in cabochons, and the correct rough orientation is important to reveal the beauty of this phenomenon. The base of cabochon should be located parallel to the plane containing the lines of inclusions’ orientation. In the case of star rubies and sapphires this plane is perpendicular to the C axis of the host crystal. Also, a punctual light source is strictly necessary to reveal this effect in all its beauty.
For a very interesting review of gems with asterism, visit the corresponding page on the Elise Skalwold’s site.