Those periods of the past when metals were unknown and stone was used as the main material for missiles, as hammers, for making tools for such tasks as cutting and scraping and, later, as spear heads. Hard, fine-grained stone was the material most suitable for flaking. Although the best locally available would have been the material of first choice, stone needed for special purposes was occasionally brought from long distances, even by early toolmakers of up to 2 million years ago as at Olduvai Gorge and Koobi Fora in eastern Africa. Flint is popularly associated with flaked stone tools, especially in Europe, but in Africa, where flint is rare, quartz, chert, and volcanic rocks, such as basalt and obsidian (natural glass), were the materials worked long before early Europeans used flint. In Europe, three Stone Ages are recognized – the Old Stone Age (Palaeolithic), the Middle Stone Age (Mesolithic), and the New Stone Age (Neolithic). In other parts of the world, different subdivisions are used. The Stone Ages are followed by the Bronze and Iron Ages. This division of prehistory into three chronological stages, defined by the main material used for tools (stone, bronze, and iron) – the Three Ages System – was first put to practical use for classifying archaeological material in Denmark in 1819. As it spread to other countries, it became necessary to subdivide the three ages. All human societies lived by hunting and gathering until the development of agriculture.