Petrographical properties of a semi-precious coaly stone, Oltu stone, from eastern Turkey

Abstract

Oltu-stone is a unique black gemstone used commonly as an ornament in Turkey. The most distinctive characteristics of Oltu-stone are its strength and its reflective black appearance after polishing. It has been widely used since the 18th century as an important material for the production of quality prayer beads. This relatively hard stone is found in marine flysch and marl deposits as scattered chunks. Macroscopically, it more closely resembles obsidian than coal.Many scientists have researched the origins of this stone, but until recently, its genesis was not well understood.Its petrographical property is remarkable and quite interesting. In order to perform petrographical studies on Oltu-stone, ten samples were taken from two locations a few kilometers from the town of Oltu which is northeast of Erzurum city in eastern Turkey. Polished briquettes and thin sections were produced from the samples for petrographical examination under a coal microscope. SEM and EDS analyses were also conducted. Internal cellular structures can clearly be traced in fluorescent mode. The great strength of the stone has always incorrectly led to the conclusion that its genesis is unlike the genesis of coal. However, fluorescent light plainly revealed detailed cellular structures. With the aid of fluorescent light, suberinite textures and silicate filling of the cell cavities were clearly observed.