Quartz pseudomorph after Feldspar. These are known as 'Gitter Quartz' (a German name for fence or lattice Quartz) or simply 'Lattice Quartz' and normally occur as a single or multiple Feldspar crystals with oriented Quartz crystals covering the sides of the Feldspar. This one however is a near complete replacement of the Feldspar (only tiny amounts of Feldspar remain when looking at the base). Fine aesthetic example with a neat gradient of crystal sizes. The Erongo Mountains is the only locality I'm aware of that produces this style of unusual Quartz specimens.