U - Z
This is odd stuff. It's a fine-grained siltstone dappled with patches and stripes rich in hematite. The formation of its remarkable banded pattern is still the subject of some debate. A number of theories have been put forward as an explanation, and the most likely to me seems to be one of dissolution while in a semi-solid state. Iron-rich layers and iron-poor layers forming as the iron 'gathers' together.
Its 600 million years old and from Kununurra in East Kimberley, Western Australia. These pieces are rough, unlike the pictured tumblestones, and roughly 6cm across.