Igneous rocks, like granite, can be extremely dense and durable, but like forged metals the strength depends on the mix of materials and other environmental factors. Some igneous rocks shatter easily or crumble at a mere touch.

It's why I've always found comparing things or people to rock or metal very amusing. The Chevy 'Like a Rock' ads are an example.

Lately, I find myself like a rock, sedentary.