Help! What is this piece of material that holds the pieces together?! Not the rivet but the lil fabric in between the rivet and the actual designed material. (Thanks for the help! :) )

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Stainless Steel Hypoallergenic Silver Rivets for Leather 50ct 6mm and 8mm Nickel-free Double Cap Rivet Studs Fast Shipping From USA 3

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I need to rivet these 3 pieces of leather together but the shafts of even my longest rivets are a little too short. I tried it and really hammered the rivets down