Made of wood. Years ago, most people packaged, purchased, and stored their sewing needles in small wood tubes. This was a very good way to safely handle sharp tiny objects such as these. 
TThese little cases are made with that principle in mind, but on a more decorative motive added. The caps are a snug slip fit. (It is fitting to mention here, that wood is a natural substance, and have been part of a live tree at some point, the plant cells and fibers, still move, swell and shrink, with the surrounding atmospheric conditions. If the fit is too tight, put the wood in a dry area, and it should loosen. If it is too loose, apply a spec of moisture, or just put it where the humidity is higher, the plant fibers will swell and snug it up.)