Jasperised wood

This polished, richly hued slice of jasperised wood is from the Petrified Forest National Park, which covers parts of Navajo and Apache Counties in north- eastern Arizona, USA. In particular, it is from the Chinle Formation, which is colourfully tinted by iron oxides. The tree from which it came, Araucarioxylon arizonicum, grew in the Late Triassic, about 225 million years ago. It was turned to stone after the forest became buried under ash deposited over wide areas by volcanic eruptions common at that time. The settled ash sealed the wood, protecting it from oxidisation and, in turn, decomposition. Silica leached from the ash, infiltrated the wood and transformed it cell by cell from cellulose to silica in a process known as permineralisation. This specimen is a cross- section of a petrified log that has been polished to reveal its beauty.

Jasper is a very fine-grained (microcrystalline) variety of silica and is often coloured by inclusions of iron oxides and hydroxides that turn it vivid red, brown, orange or yellow.

This specimen was acquired in 1984 for the Planet of Minerals exhibition that opened in 1986. The purchase was arranged by Albert Chapman when he was at the famous Tucson Gem and Mineral Show in the USA in 1984.