Actually ec, your rules of thumb can be confusing. Cast iron in my experience is generally markedly ferromagnetic. I suspect that you have had to do with highly austenitic cast iron, perhaps rapidly quenched. I seldom encounter cast iron nowadays, but it used to be quite common in some building applications for example, and I suspect that those castings were more gently cooled, whether deliberately to reduce brittleness or because the moulds were not generally opened so quickly, I could not say.

It can be very difficult to guess the magnetic nature of substances (especially nowadays, with so many changes in the preferred materials for everyday objects) and even some bronzes contain enough iron to be magnetic, so watch it!

Agreed fully. I am wrong because I left out "really strongly" magnetic. I was basing  my obervation on seeing how experienced repair welders determine what material they are dealing with. They would grind a flattish surface (if they were allowed)  on the material and with a magnet "feel" if it was iron or "steels". I also suspect that the "feel" is not linear to the curves below because they  (the welders) were always quite certain of their diagnosis.even with non flat surfaces.

http://en.wikipedia.org/wiki/File:Magnetization_curves.svg

The link above demonstrates cast iron is roughly half of "steels"

Thank you.

I am sure that those welders were perfectly competently plying their craft and using the appearance "feel" and sonic effects as well as other behaviour characteristic of metal types, apart from magnetism, whether they were aware of it or not. Particular metals tend to get used in particular types of applications, so the selection of possibilities is usually rather small, which is why such "rules of thumb" generally are quick and effective in practice.