radiometric dating

There are a number of different ways, but generally you test for different isotopes to determine their decay state. Farmerman will be able to provide more details.

Are you asking about all rocks in general or about a specific rock you have found?

I find it easier to do with stones, rather than rock.

Mick is reckoned to be anything between 69 and 122, and whereas Keith is about the same age, he appears slighlty older due to erosion.

Charlie Watts is reckoned to be slighly post Jurassic, and Ron Wood is currently being dated as I write.

Its difficult depending upon the rock.

The hardest are metamorphic rocks, do you know why?