How sand can change a fossil

"So how did this magnificent opalized tree came to be? Initially, water filled the cracks and empty spaces of the source fossil, and the silica content in the water hardened and turned into opal or chalcedony."

Sand (silica)

https://www.zmescience.com/other/geopicture/geopicture-of-the-week-tree-fossil-with-opal-growth-rings/