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Brief Description: Obsidian is natural black glass and comes exclusively from volcanic areas of a geologically younger age. Obsidian belongs to the acid volcanic rocks. These rocks often have porphyritic texture, while there are also varieties with glassy texture. The glassy texture is due to the high concentration of silicon dioxide SiO2 in the magma, resulting in high viscosity that prevents crystals development. This natural glass, though, is unstable and falls into crystalline state with the passing of geological time. Its hardness is 5 ? 5.5 and its specific gravity is almost 2.6. It has glossy black colour and conchoidal fracture. When the acid magma cools very fast, then rocks emerge, which consist almost exclusively of glass, like obsidian and pumice. Obsidian consists of less than 1% water by weight, because magma has rather high temperatures (750-950°) during its infusion into the surface of the Earth and cannot hold water. Obsidian, due to its composition, its rather high hardness and toughness, but also its easy processing, was already used by the end of the Upper Palaeolithic period for the making of stone tools (sharp blades that served as knifes, curettes and razors). Obsidian deposits in eastern Mediterranean are very few and are located in some Aegean islands, like Milos, Antiparos, Nisyros and Gyali. Obsidian from Milos was used due to its hardness by Aegean inhabitants, in the Stone Age and Bronze Age, for the making of tools and weapons (arrowheads). On the contrary, obsidian from Gyali ? which is more fragile and cannot take fine processing ? was used in the Late Bronze Age for the making of stone pots. Obsidian, apart from the making of tools, was also used for ornamental purposes, because it has the property of presenting a different appearance depending on the way it is cut. The processing towards a specific direction it has a beautiful black colour, while the processing towards the opposite direction produces flakes of grey colour. 
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