Feature Gems & Gemology, Winter 1984, Vol. 20, No. 4

An Extraordinary Calcite Gemstone

A 1,156-ct calcite gemstone cut from a twin crystal presents an unusual phenomenon when the interior is viewed through the table. One observes a multitude of colored images of the lower pavilion facets that change position and color with change in the viewing angle. This kaleidoscopic effect results in part from dispersion and the high birefringence of calcite, but the large number of facet images is caused by the twinning. As a single light ray enters the table, it is broken into two rays, O and E. On crossing the twin plane, each of these rays is divided into O and E rays, resulting in four rays that are reflected across the pavilion to be reflected toward the table. On this upward path, the rays again cross the twin plane and once more are divided into new O and E rays. Thus a single entering light ray emerges as four O and four E rays.