Swizzle:
Go to a discount store or a crafts store and look for a sheet of flexible poster board. They come in white, red, blue, and black. Get a sheet of the black to use as a background for photographing your marble finds. White would be too bright with flash usage, and the other colors might "bleed" to the marbles. Black would not reflect the camera's flash and should act as a light absorber and allow the marbles' colors to be "pure".
Use the sheet by placing it on a table; place the marbles on it, far enough away from all edges so the edges won't be in the pix. Then, lift the back half of the sheet into the vertical to form a backdrop. Prop it up with some books or other objects with enough weight to keep it from being pushed away by the poster board. Some simple masking tape can be used to temporarily hold the poster sheet in place for your photo session.