Obsidian. It would be brought from Cappadocia, 13 days walking from where it was found.

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. The first attested civilized use is from excavations at Tell Brak (northeastern Syria) dated the late fifth millennia. Obsidian was valued in Stone Age cultures because, like flint, it could be fractured to produce sharp blades or arrowheads.

Correspondent: J.M.Criel, Antwerpen.
Source: (amongst others) Wikipedia .