Endodontic fiber posts: glass vs quartz. Can you explain composition and manufacturing differences?

Different material. I have not heard of a post being made out of glass, or quartz. There are metal post or what is know as fiber post which are used when you don't want to use metal.
See below. Relyxtm fiber posts are made from glass fibers embedded into a composite resin matrix.Luscent anchors®a unique fiberglass resin post and core system.The glass fibers contain silica similar to the fillers of composite materials.Older fiber posts had carbon fibers. Different posts have different number of fibers effecting flexural strength and fatigue resistance. Materials also effect color of post.