Hydrothermal rocks are created when minerals precipitate from hot water and crystallise on the inside of cracks or hollows in other types of rocks.  This can create veins of silica-rich stones that are often suitable for flaking into tools.  The type of stone depends on the water temperature, the rate of water flow, and pressure.  

Water percolates downward through cracks and pores in the earth’s crust.  The water is heated as it moves deeper and dissolves silica from the rock it travels through.  When silica-saturated water rises again, it cools and and the silica is precipitated in cracks and hollows.  Uplift and erosion eventually exposes these silica-rich stones as seams and nodules at the earth’s surface.