1911 Encyclopædia Britannica/Sunstone
SUNSTONE, a felspar exhibiting in certain directions a brilliant spangled appearance, which has led to its use as an ornamental stone. The effect appears to be due to reflections from enclosures of red haematite, in the form of minute scales, which are hexagonal, rhombic or irregular in shape, and are disposed parallel to the principal cleavage-plane. These enclosures give the stone an appearance something like that of aventurine (q.v.), whence sunstone is known also as “ aventurine-felspar.” It is not common, the best-known locality being Tvedestrand, near Arendal, in south Norway, where masses of the sunstone occur embedded in a vein of quartz running through gneiss. It is found also near Lake Baikal, in Siberia, and at several localities in the United States, notably at Middletown, Delaware county, Pennsylvania, and at Statesville in North Carolina. The felspar which usually displays the aventurine appearance is oligoclase (q.v.), but the effect is sometimes seen also in orthoclase (q.v.): hence two kinds of sunstone are distinguished as “ oligoclase sunstone ” and “ orthoclase sunstone.” The latter has been found near Crownpoint and at several other localities in the state of New York, as also at Glen Riddle in Delaware county, Pennsylvania, and at Amelia Court House, Amelia county, Virginia.