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What happens when a tree limb is covered by hot volcanic ash? The volcanic ash might partially or fully burn away the limb. When that happens, a cavity can be created inside the ash, and, over millions of years, groundwater deposits minerals in the cavity to create a cast of the original limb. Fine details of the original limb, like bark texture, can even be retained by the fine volcanic ash.
This 50 million years old conifer limb was cast in chalcedony. It was found in Tom Miner Basin in Montana. It measures 200x170x12mm.