Aventurescent Gemstones

The phenomenon, aventurescence, is seen as a strong spangled reflection of light from plates or flakes of another mineral included in the stone. This effect is seen most often in aventurine quartz, sunstone (feldspar) and the man-made aventurine goldstone.

Aventurine quartz, which gives the name to this effect, is actually not a true mineral, but a rock known as quartzite. It is composed of a granular interlocking mass of quartz crystals. When quartzite contains small mica plates that reflect light, it is then termed aventurine. Usually the included mica is a green, chrome-rich type called fuchsite. It is these mica plates which produce the spangling reflection and give the green color to the stone. The mica may also be of other types, in which case the stone would be other than green. Reddish brown, gray, creamy white, and bluish white are some of the colors it has been found in, but the majority of the aventurine seen in jewelry is of a green color.

Sunstone is the name applied to the varieties of feldspar which show the phenomenon of aventurescence. It may be either oligoclase or labradorite in composition and owes is sparkling sheen to included plates of hematite or goethite. The plates of a red, orange or green color are arranged in parallel rows and they give their otherwise colorless host a rich orangy red color and sparkle.

The best example of aventurescence is seen in the man-made aventurine glass, goldstone. It is a soda-lime glass colored reddish brown by cuprous oxide. With subsequent treatment, the cuprous oxide crystallizes out as thousands of flat hexagonal or triangular plates of crystallized copper. Goldstone is now manufactured in other colors as well.

Cloudy pieces of amber are often heated in rape seed oil to remove the cloudiness. The oil, which penetrates the stone and fills the air spaces causing the cloudiness, also causes stress fractures, which are termed ‘sun spangles’. These may sometimes be present in sufficient numbers to give an aventurescence to the stone.

With the possible exception of sunstone, which is occasionally faceted, most aventurescent gems are cut as cabochon. The plate-like inclusions which give rise to this effect are generally oriented parallel to a certain plane. To obtain the best effect, the base of the cabochon should be parallel to this plane. Maximum sparkle is obtained by cutting the crown with a very low dome.