Review (4/5): This appears to be a polyurethane material
This is not a braided multi-strand elastic that you might see on surgical masks. Rather it is a solid "rubber" material embossed with a pattern to resemble the traditional material. Technically I would call it an "elastomer", and it feels a lot like polyurethane. It is quite stretchy and will expand to roughly 250% of its non-stressed length, with very modest force. We ended up using it for several masks, but switched to the more traditional braided elastic once that finally arrived. The braided material has an inner elastomer core that is overlaid by a cotton braided covering (much like insulated wire), and then multiple strands of that are joined together side-by-side to form a ribbon. So this stuff will work if you can't get the real thing, but I might be concerned about how long it will last under repeated usage.