How Does a Tree Turn Into Stone? Molecule by Molecule

Wood buried rapidly and sealed from oxygen does not decay, and groundwater carrying dissolved silica percolates through it over a long period. That silica precipitates into the empty spaces first, filling cell cavities and the channels that carried water in the living tree, which supports the structure and preserves its shape. The organic material of the cell walls then breaks down slowly and is replaced in place by further mineral, so the arrangement is preserved rather than merely cast. Because the process works at the scale of individual cells, a thin section of the result shows cell walls, growth rings, ray cells and in exceptional cases the internal structures of individual cells.

The spectacular colours come from trace elements in the mineralising water:

• •Iron oxides giving reds, oranges and browns
• •Iron in a different form giving yellows
• •Manganese giving pinks and purples
• •Copper, chromium and cobalt giving greens and blues
• •Carbon remaining, giving black
• •Pure silica with nothing else, giving white and grey

The preservation is detailed enough that these specimens are studied as botany rather than merely as minerals. Wood anatomy differs systematically between plant groups, so a thin section identifies what kind of tree the specimen was, frequently to family and sometimes further, which supplies evidence about ancient forests that leaves and pollen cannot. Growth rings record the seasonality of the climate, with strong rings indicating a marked seasonal contrast and absent rings indicating tropical conditions, and measuring ring widths across many specimens reconstructs climate variability tens of millions of years ago. Damage from insects, fungi and fire is preserved. And the position of standing trunks shows where a forest actually grew.

The process that preserves wood preserves other material by the same route and the results are equally informative. Bone is infiltrated and replaced in the same manner, which is why fossil bone shows internal structure and why thin sections of it can be studied. Whole animals are occasionally preserved in silica at cellular detail in particular deposits. Fossil cones, fruits, seeds and even flowers survive where the same chemistry operated. Coal balls are lumps of mineralised plant material within coal seams that preserve the tissue of the swamp forest in three dimensions, which is how the anatomy of those plants is known. And silicified bacterial mats in very ancient rocks supply some of the earliest direct evidence of life.

A small number of localities produce material in extraordinary quantity and quality. The Arizona deposit preserves enormous trunks from around two hundred and twenty million years ago, brightly coloured and abundant enough that a national park was created in 1906 specifically because collectors were removing it by the wagonload. Deposits in Argentina preserve trunks of great size. Greek, Egyptian, Indonesian and Australian localities are substantial. Antarctic deposits preserve forests that grew at high latitude under conditions with months of darkness, which is among the more striking evidence for past climates. Collecting from protected sites is prohibited and enforced, and material is traded legally in quantity from unprotected localities.

Wood buried without oxygen is infiltrated by silica-bearing groundwater that fills the cell cavities first and then replaces the cell walls in place, preserving structure down to individual cells. Trace elements in that water produce the colours. Thin sections identify the plant group, growth rings record climate seasonality, and damage by insects and fire survives, which makes the material evidence about ancient forests rather than only a decorative stone.