Index to Quartz Digging Cleaning Worth Fee Pay Mines Types Forms Inclusions Geology Mineralogy Synthetic Gemstones Handedness Experiments

Asteriated Quartz Webster defines asterism as an optical phenomenon of some crystals that exhibit a star-shaped body produced by reflected light or by transmitted light and gives star sapphire and mica as examples. You may be familiar with some of the well-known gems, such as ruby and sapphire, that produce 6-rayed stars when cut as cabochons. They are both a form of corundum, a mineral that crystallizes in the Hexagonal crystal system. They owe their star-producing property to microscopic inclusions of other minerals that are oriented parallel to the 3 a crystallographic axes. These inclusions appear to have exsolved from the mineral as it cooled down after crystal growth. The process of exsolution occurs because at high temperature the mineral's lattice structure is capable of allowing various elements to be incorporated, but after the mineral formed and while it cools, the lattice structure forces these materials out of the structure. So exsolution is responsible for producing the oriented nature of the included mineral. Now, asterism is not very common in most varieties of quartz, but is sometimes seen in rose quartz. Rose quartz deposits are most often associated with granite intrusions and tend to have formed in pegmatite bodies at higher temperatures than vein quartz. Rose quartz typically has oriented needle-like inclusions of rutile, the mineral having formed by the process of exsolution. Spheres cut from rose quartz containing exsolved rutile may show marked asterism. Looking directly down the c crystallographic axis one will see the strongest set of rays. Asterism is best observed by using a single point light source, like an incandescent bulb or the sun. Under this type of lighting, rotate the sphere around to see if it produces a star (rotating allows you to find the proper orientation). For a clear mineral, like in the photo of the rose quartz sphere, placing a mirror directly underneath the specimen results in a strong star that appears to hover slightly above the sphere! In quartz the scattering of light along the oriented rutile needles causes asterism. The smaller the rutile needles the sharper the star effect. Rutile needles in rose quartz usually are 0.01 to 0.02 micron in thickness and may range from a fraction of a millimeter to over a centimeter in length. If the needles are visible to the eye as inclusions, the quartz is milky with a low translucency and lacks asterism. I have heard of a few specimens of rock crystal (colorless quartz crystal) that possess this phenomenon, but have been unable to obtain one for my collection so far. The only example I have is a single rose quartz sphere that I obtained in a trade with a dealer several years ago. Unfortunately, I do not know its origin, except that he purchased several asteriated rose quartz spheres at the Tucson Show. I suspect if I ever get a clear piece of rock quartz that exhibits this property, it will come from a pegmatite body like rose quartz, not from hydrothermal vein quartz like we have here in Arkansas.   Contact the authors of Rockhounding Arkansas revised September 1999 ©Rockhounding Arkansas 1998 http://rockhoundingAR.com