Conductivity of ferrite n... What is the function of its core in RFC(?)

- I tested conductivity of some ferrite cores, found some conductive (used in transformers) and found some non-condictive (used in RFC). Actually what is true?
- What is the function of ferrite cores in small RFC? Just to increase inductance or...? In resistor type small RFC, the core of ferrite is just to increase inductance? or for resistance too? Is that conductor or semi-conductor or non-conductor?

Ferrite is generally used for magnetic purposes rather than conductive purposes.
Ferrite can do various things for a coil, usually it increases the inductance which means a coil can be smaller and have less series resistance and less cost.
Also some ferrites are very lossy at radio frequencies so you can use these to absorb rf interference.