In stone age obsidian was used as a cutting tool for making any sharp tool and it is still used as a cutting tool in modern surgeries. Obsidian is used as a cutting tool because of its conchoidal fracturing where it breaks into thin sheets and have sharp edges. Modern archaeologists have developed a relative dating system, obsidian hydration dating, to calculate the age of obsidian artefacts. It was also polished to create early mirrors. Like all glass and some other types of naturally occurring rocks, obsidian breaks with a characteristic conchoidal fracture. Obsidian was valued in Stone Age cultures because, like flint, it could be fractured to produce sharp blades or arrowheads. The first attested civilised use is from excavations at Tell Brak dated the late fifth millennia. Anatolian sources of obsidian are known to have been the material used in the Levant and modern-day Iraqi Kurdistan from a time beginning sometime about 12,500 BC. Use of obsidian in pottery of the Neolithic in the area around Lipari was found to be significantly less at a distance representing two weeks journeying. The first known archaeological evidence of usage was in Kariandusi and other sites of the Acheulian age (beginning 1.5 million years BP) dated 700,000 BC, although the number of objects found at these sites were very low relative to the Neolithic.