Some Bowyers call it Phenolic, some call it Linen Micarta. Hope this helps.
"Phenolic resins are found in myriad industrial products. Phenolic laminates are made by impregnating one or more layers of a base material such as paper, fiberglass or cotton with phenolic resin and laminating the resin-saturated base material under heat and pressure."
Charlie, this process would give you the "grain" you're seeing in the riser. I have a Toelke Bowbolt and you can really see it in that riser.