Its petrified wood. Its a type that is a common opal infill. Its quite common from Jurassic nd Cretaceous beds of the North American west and Africa . Some of the beds, where hot water stayed and penetrated , fire opals are formed . Its quite common to have this dull massive woodlike rock showing layers and rings and then, BAM, youd see whole lenses of fire opals in the wood.
Id look round the same area because that wood structure can houae some really nice gem quality opal if there was enough of a hydrothermal contact zone.