The Donnersberg area, a famous vulcanic region in Palatinate, is the origin for a very interesting combination between petrified wood and agate. The wood was completly changed into carbon by hot vulcanic ashes before silification. The wood losses substance and shrinked, so that a lot of fine cleavages did show up. While silification red chalcedony filled up this cleavages. Petrified wood with visible cell structures is very rare at this locations.
More information about silificied wood is given from Robert Noll at “Die Kieselhölzer der Donnersberg-Formation”, Catalogue of the 9th “ Inter- nationalen Achatbörse “, 2009