Tiger ironstone showing tigereye, close-up. Tiger ironstone is the popular name for a metamorphosed gemstone found in very old rock formations. Over 2 billion years ago, hematite, jasper and tigereye were slowly forced together as the tectonic plates shifted, creating this intricately banded rock. The central fibrous mineral is tigereye (granular quartz), with the golden lustre of its yellow and brown stripes reversing when viewed from the opposite direction. Its fibrous nature is due to the fact it was originally composed of crocidolite fibres, which were then replaced with silica. Dendritic patterns of impurities (either manganese or iron oxides) can be seen. This piece came from West Australia. Magnification: x2.9 when printed 10 centimetres wide.