Hey, I'm sorry to add this to the post but it's important, I am a physicist and a science communicator, so when I saw this mistake it was important that I make it clear.

You say:
what is susceptibility of rocks?
This simply means where their is no external field i.e the rock sample has zero net field.

This is not the answer, and really has no meaning. Sorry.

You need to read a bit about magnetic susceptibility in order to correctly describe what it is.

Instead of educating you, I implore you to read about it yourself.

Wiki is not the best place for scientists but keeps things fairly simple, so i invite you to look here: https://en.wikipedia.org/wiki/Magnetic_susceptibility

If you google "magnetic susceptability" it states:
Magnetic susceptibility is a dimensionless proportionality constant that indicates the degree of magnetization of a material in response to an applied magnetic field. A related term is magnetizability, the proportion between magnetic moment and magnetic flux density.

Can you see the big difference. When you write science articles and share to people, as scientist it's important to give correct and accurate information. So next time do a bit of research and make sure what you are saying is correct. It will help with gaining personal knowledge, teaching others and getting good upvotes also.