Obsidian is ancient volcanic glass and was once widely used for arrowheads. This volcanic rock was called pumax by the Romans. Pumice is used for polishing hard things. Nobody seems to know where the German word came from. Quartz is an abundant mineral.
Most sand is made up of small pieces of quartz.
Quartz is important in glass making and in electronics. It is also used for sanding hard surfaces. It is used as a building stone. Slate is a metamorphic rock formed from shale under great pressure. It is used for chalkboards and as a building stone.