Granite feels really cold so it must have a high thermal conductivity. The value I found says from about the same as yesterday's slate up to 4.0 W/m K. This felt colder than the slate which means maybe at the higher end. I tried to find out why stone conducts heat quite well but found nothing obvious. It may be that the substances that don't feel as cold benefit from trapped air (eg carpet). If there were a lot of free electrons, it would conduct heat well. I will have to try to measure the conductivity.