[LeifCarrotson]

You'd have kept your house if you had a reasonable judge, but only if the statement was "rock is more like an insulator than a conductor" and not if the statement was "radio will work in a mine deep underground".

Everything is an imperfect conductor. There's just an awful lot of rock in a mine, and in practice it's often mixed with highly conductive mineral-bearing water.

Looking up some numbers, "rock" has a conductivity on the order of 10^-5 sieverts per meter. That's billions of times less conductive than something like copper, and comparable to something like wood, which we think of as an insulator. You could touch a piece of wire energized with high-voltage AC to a granite or quartz countertop, touch the countertop a short distance away with your hand, and connect your other hand to the ground reference and feel no shock at all.

I suspect that someone was taught that radio doesn't pass into a faraday cage or escape a microwave because the conductivity of the metal mesh absorbs the radio waves, and separately that radio does not work underground, and synthesized those facts to "rock is a conductor" without actually getting out a 4-wire Kelvin probe and measuring the conductivity of the rock.