Pietersite is a brecciated form of the same fibrous material that makes tiger eye, broken apart and recemented by silica, which is why the fibres run in every direction instead of in neat parallel bands. That is what gives the rolling, stormy flash. It was first found in Namibia in the 1960s and later in China. At around 7 on the Mohs scale it is hard-wearing and safe in water. Hand-picked in St Andrews and posted UK-wide.