A type of hard stone, often gray in color, found in rounded nodules and usually covered with a white incrustation. A member of the chalcedony group of water-bearing silica minerals, it was found from early use to fracture conchoidally and was ideal for making stone tools with sharp edges. It is chemically a quartz, but has a different microcrystalline structure. It can therefore be flaked readily in any direction and so shaped to many useful forms. It occurs widely, and where available was the basic material for man's tools until the advent of metal; it is commonest 'stone' of the Stone Age. The only types of stone preferred to it were obsidian and the tougher rocks used for ground tools in the Neolithic. The term is often used interchangeably with chert and also as a generic term denoting stone tools in the Old World. Nodules of flint occur commonly as seams in the upper and middle chalk of northwest Europe. During the Neolithic and Copper Age of Europe, flint workers recognized that flint from beds below ground were of superior quality to surface flint, especially for the manufacture of large tools such as axes. These beds were exploited by sinking shafts and then excavating galleries outwards. Flint mines are known from many areas of Europe and good examples occur in Poland (Krzemionki), Holland, Belgium (Spiennes) and England (Grimes Graves).