They are rayon. Most of them are okay. Some of them are poorly wound but at about 25 cents a spool who cares. There are 350 of them for 80 so you can find another color really close. I have one color that won't work at all but most of them are fine. I have Mettler and Guterman also and I have had breakage with them too. I doubt there is a color I don't have. Now I can take my time and build up a supply of large cones at my leisure. You can do a lot of work with all this thread. I also got the bid on 100 metalic threads and the rayon is similar in color and sheen. I havn't used them yet so can't say how they are.