What is the material of the stone mill used to grind beans in the countryside?

Generally with granite stone quality, granite is a kind of y acidic igneous rock. Commonly known as granite. Silicon dioxide content is more than 70%. The color is lighter, with off-white, flesh red more common. Mainly composed of quartz, feldspar and a small amount of black mica and other dark minerals. Quartz content of 20% to 40%, alkali feldspar more than plagioclase feldspar, accounting for about 2 / 3 of the total feldspar. Alkali feldspar for a variety of potassium feldspar and sodium feldspar, plagioclase feldspar is mainly sodium more feldspar or more feldspar. Dark minerals are dominated by black mica, with a small amount of hornblende.