Stone under the microscope – Black Galaxy

Black Galaxy is the trade name given to a generally medium-grained blackish stone that displays small, conspicuous reflections. Because of the rarity of stone with such reflective features Black Galaxy is perceived as a prestigious product. Its main applications are internal but there is an increasing tendency to also use this stone externally. It emanates from near the village of Semakurthy in the Ongole region of Andra Pradesh in southern India. Of the 30-40 quarries from which this stone type is extracted only about 10-12 contain stone of export quality. The proliferation of quarries is attributable to the low relief, sheet and residual boulder formation of the stone.

A polished slab or tile of Black Galaxy shows many of the characteristics of this granite including colour, texture, and structure but resin applied to the samples inspected conceals the degree of alteration and true character of finish. The colour is predominantly a black with conspicuous flakes of a bronze-coloured mineral providing a sparkling effect depending on the angle of light reflectance. There is a degree of granularity because many of the bronze-coloured crystals are of similar size and have a similar degree of reflectance. Some variation in the abundance of reflective bronze-coloured crystals on different sides of a thick stone suggests a structural or magmatic flow orientation. Contrary to popular belief the sparkling effect is not due to the presence of large flakes of mica.

The surface of the tile contains a multitude of microfractures which is probably the reason for the resining process in order to conceal some of these physically undesirable features. There are also small pits and a surprisingly high abundance of sulphide. The darkness of the rock effectively conceals any alteration that might be present.