I think the above may have been derived from a comment I made but needs slight modification.

In the majority of instances, a rug with over 400 KPSI will probably have a decent grade+ wool or silk pile. Foundations vary.

This was a loose rule of thumb I mentioned because of the implementation of simulated silk in many weaving. Simulated silk comes in several forms, rayon, mercerized cotton, etc. In 90% of cases when questioning whether a silk pile is real silk, you can pretty much rule out synthetic fibers for 400 KPSI rugs or more. don't forget, rugs from Turkey (specifically kayseri), Kashmir and China are the most common exceptions, but generally speaking, it would be counter productive to use faux silk in a 400+ KPSI rug. It's really best to just do a test by pulling a knot from the back of a rug.