It’s been decades since I cracked my last geology textbook, so I might not have my terminology just right- but I think I’m close on this one. I’m going with metamorphic rock, with a vein of quartzite bearing an inclusion of another rock type. Now it could also be that the heart is so-called country rock, and that the quartzite vein is in a sense the inclusion in the larger mass of rock that surrounds it.
But what it really boils down to is this: it’s a found heart. Right?