These 4 inch diameter acrylic plastic craft rings come 12 pieces per package. They are 5/16 inches thick. They are made of a solid hard plastic material. They so have air pockets inside the rings so do not think the rings are defective. They are designed that way. Call it: plastic rings, craft rings, plastic rings for crafts, acrylic rings, clear rings. These plastic craft rings are a great general purpose item used for various projects. We carry these rings in 3, 4 and 5 inches.