Arizona's petrified trees are solid quartz, and thieves mail them back with apology letters

The fallen logs scattered across Petrified Forest National Park, in Arizona, look like wood, but they are almost solid quartz. The trees grew between about 210 and 227 million years ago, in the Late Triassic, and over millions of years their wood was replaced, cell by cell, by silica, the same mineral that makes up most beach sand and every amethyst.

It all began with a lucky burial. According to the National Park Service, dead trees washed into a large ancient river system and were covered so fast and so deeply by mud, sand and debris that oxygen was cut off. Without oxygen, the fungi and bacteria that normally rot a fallen trunk could barely do their work.

Volcanoes to the west supplied the second ingredient. Their ash, rich in silica, drifted into the region and mixed with the river sediments. Groundwater dissolved that silica and carried it into the buried logs, where it seeped into the cells and slowly took the place of the organic material as it broke down.

The park's Trees to Stone guide describes two stages. First the silica turned the wood into opal, a step that took only thousands of years. Then, as the logs sank deeper, the opal lost its water and recrystallized as quartz, which took millions. Scientists date the forest not from the wood itself but from zircon crystals in the volcanic ash around it, which hold uranium that decays into lead at a known rate.

Pure quartz is colorless, so the reds, yellows and purples come from impurities. An older NPS guide explains that iron minerals produce mustard, orange, red and black tones, while manganese produces blue, purple and brown. A 2016 study in Geosciences by George Mustoe and Marisa Acosta measured trace metals in 35 samples of fossil wood and found that iron explains most colors, while chromium is behind the rare bright green pieces from Arizona.

The quartz also explains a strange detail: many logs look as if someone sliced them with a chainsaw. Starting about 60 million years ago, the slow uplift of the Colorado Plateau put the still buried trunks under enormous stress. Because quartz is hard but brittle, they snapped like glass rods, in clean breaks spaced along the trunk. Erosion later stripped away the rock above and brought them to light.

Most of the trees have been given the scientific name Araucarioxylon arizonicum, an extinct conifer. The area was protected in 1906, in the second group of national monuments created under the new Antiquities Act, and it became a national park in 1962. Today, taking even a small chip of wood from the park is against the law.

Some visitors take pieces anyway, and a surprising number later send them back. Since 1934 the park has received what staff call conscience letters: packages with stolen fragments and notes blaming the rocks for car trouble, illness, deaths in the family and other misfortunes. The archive holds more than 1,200 of these letters, according to Collectors Weekly, and the artist Ryan Thompson published a selection in the book Bad Luck, Hot Rocks.

The returned stones never go back to the landscape. As Smithsonian Magazine explained, even letters with detailed maps do not help, because scattering a piece of unknown origin could spoil that spot for research. Instead, the fragments end up on a conscience pile beside a service road closed to the public.

Sources: National Park Service: Petrified Wood; NPS: Trees to Stone (2019); NPS: Trees to Stone (2011); Mustoe and Acosta, Geosciences (2016); NPS: Frequently Asked Questions; Collectors Weekly; Smithsonian Magazine. Cover: Petrified log at Petrified Forest National Park.