Trying to measure the resistivity of granite

Having noted that granite feels cold ("stone cold") so must conduct heat quite well, I decided to see whether I could measure the resistivity and thus the conductivity of the granite. It didn't work

I taped one lead to the bottom of the worktop and the other lead to the top. The reading was off the scale and thus larger 2 million Ohms. I tried the connections taped together on the top and got the same thing

I did check that it wasn't a broken lead by clipping the leads together. The leads have a resistance of a couple of Ohms. A quick search suggests that granite can have very variable electrical properties. I suppose that reflects the different compositions, even in Cumbria. I found one record of granite having a resistivity between 25 and 1500 Ohmmetres. The work top have an area of about 2 square metres and a thickness of 0.03 metres. That would give a maximum resistance of about 20 Ohms. Perhaps the contacts were poor.