Biblio. IMN

'Mercedes-star' quartz contains abundant long, narrow, randomly oriented rutile inclusions, and is notable for its unusual asterism. In particular, it displays weak three-rayed stars, with rays that meet at the centre as in the Mercedes-Benz emblem. In the classical theory of asterism such stars cannot exist, and they have not been seen in any other gem materials. We examined three samples (two spheres and one egg shaped) of 'Mercedes-star' quartz from Brazil. Ideally, this material displays six three-rayed stars located along two latitude planes parallel to the equatorial plane (perpendicular to the c-axis). In addition, six faint four-rayed stars are located along the equatorial plane. Therefore, in total, this quartz may display 18 stars located on three coaxial circles. The rays of all the stars consist of a path of reflective segments of unknown illumination origin, but which appear to result from double reflection of the light beam, first by Brazil-law twin planes and then by the rutile needles, making the phenomenon uncommon. In addition to the abovementioned stars seen in reflected light (epiasterism), more-transparent 'Mercedes-star' quartz may also show two six-rayed stars with transmitted light (diasterism) when illuminated from behind and viewed parallel to the c-axis.