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A very good millstone is generally rich in silica: the higher the percentage, the stronger the rock, silica being the hardest common mineral on the Earth's surface. The same is true of sandstone with siliceous cement, where the percentage of silica is high because both the grains and the cement are siliceous in nature.
The Mühlsteinbrüche ("Millstone Quarries") south of Jonsdorf in the Zittau Mountains in Saxony are a region of bizarre rock formations, which have been formed by the quarrying of sandstone for millstones and also by weathering processes. They are a popular hiking and climbing area.
The natural condition for the quernstone and millstone production in the area is the rock type garnet mica schist. The quarries are located from the shoreline up until approx. 200 m above sea level, but a majority is situated less than one kilometer from the sea and closest harbor.
However, such rocks are not always available, meaning that quern-stones have been manufactured from a wide variety of rocks, including sandstone, quartzite and limestone. Quernmore Crag near Lancaster in England is named after the quarrying of millstone grit used to make quern stones in these parts.
The larger part of the national park is formed in sedimentary rocks from the Carboniferous period (359 - 299 Ma). In stratigraphic sequence i.e. youngest at the top, the sequence consists of: [3] Pennine Coal Measures Group; Millstone Grit Group; Yoredale Group (Asbian - Yeadonian) Stainmore Formation; Alston Formation (inc Gayle Limestone ...
The term "Millstone Grit" was also adopted in South Wales where rocks of similar age and lithology are found though the Millstone Grit Series of this region has recently been formally renamed by the British Geological Survey as the Marros Group. The thickest bed of sandstone within it was known as the Basal Grit and this has now been renamed as ...
Quartz dolerite or quartz diabase is an intrusive rock similar to dolerite (also called diabase), but with an excess of quartz. Dolerite is similar in composition to basalt, which is volcanic, and gabbro, which is plutonic. The differing crystal sizes are due to the different rate of cooling, basalt cools quickly and has a very fine structure ...
The combined Roaches Grit and Ashover Grit are amongst the most widespread sandstone units within the Millstone Grit Group of the Peak District. Along with other similar sandstones, such as the immediately overlying Chatsworth Grit, it is assigned to the Marsdenian sub-stage of the Namurian stage within the Carboniferous period.