Petrified wood has a thick patina of hydrated silica. highly agatized material. See beyond the patina of the grains bleaching of low PH soil & water. The white is wood grain.
Brazilian agate was shipped to Seattle WA. in 1962. This material is river worn. Thick patina of hydrated silica. The white skins, material shows a thick patina silica. All Brazilian agates are hollow or soiled geodes. depending on cavity fill.
All slabs are cut to about 1/4 inch. or 5-6 turns on a highland-park saw. Shop help does (ME) thicken some slabs a bit. 7-10 turns. depending on the obsidian material.