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Aventurescence is an optic phenomenon observed in some gems when inclusions create a pattern of brilliant flashes and color spots within the host mineral. Just as with chatoyancy and asterism, inclusions responsible for this effect are usually formed by exsolution process.
Some common examples of gems with aventurescence are feldspar group sunstones, full of small flat exsolution inclusions of hematite or ilmenite, producing bright reflections of light and rainbow interference colors. Similar effect can be seen in so called iolite sunstone, caused by the same type of inclusions. The Oregon sunstone constitutes a particular case of labradorite feldspar with metal copper inclusions responsible for aventurescence. The term “Schiller effect” is frequently used for this particular gem and type of aventurescence.
15.54 ct Oregon sunstone with copper inclusions, cut by John Dyer, photo Lydia Dyer, www.johndyergems.com
Aventurine quartz contains green mica flakes (fuchsite) providing overall greenish color and bright reflection spots to this gem; however in this case inclusions are syn- or protogenetic and not formed as a result of exsolution.
Green fuchsite inclusions in aventurine quartz. Field of view 3 mm.
A very common imitation of this natural effect can be observed in “aventurine glass”, also known as “goldstone”. Coming in different colors and with bright flashes caused by metal copper inclusions, it was invented in Venice in the XVII century and is still very popular in imitation jewelry. Curiously, the gemological term “aventurescence” comes from the Italian name of this artificial glass, avventurina, so it is probably the unique case in gemology when imitation gives the name to some natural stones.
Aventurine glass or goldstone, polished beads (left) and copper inclusions seen under magnification (right, field of view 2.3 mm).