These are ferrite, the other type is iron powder.
I found this site useful for unknown cores.


http://www.alan.melia.btinternet.co.uk/toroids.htm



"Graham" wrote in
:

I am trying to make some high value toroids, about 1mH. I have a
commercial one using a half inch core with about 20turns on it to give
3.3mH. Does anyone know what the core material/description is or an UK
source please? The normal ones such as T37-2s etc only give a few uH.

Thanks

Graham