In my setting, among various other magical creatures, gargoyles exist. They're winged, horned, tailed humanoids (if you picture the Disney show, you'll have an accurate enough mental image to work with) that only become flesh once a month, during the 24 hour period humans don't know about that happens when the moon reaches peak fullness, where time completely stops for all non-magical creatures and the world is flooded with magic. This special "Moontime" magic is the only type of magical energy that gargoyles can absorb to sustain themselves, and so at all other times they enter a state of hibernation where they turn to stone until the next full moon, resembling statues in the meantime.
Being so incredibly helpless for the vast majority of their lives, they naturally rely heavily on alliances with their fellow magical creatures for protection from being damaged or destroyed while they're made of stone. However, that doesn't mean it wouldn't help a lot to know how to make themselves the least "smashable" they can be. So what follows is the first of a few questions I plan to ask regarding gargoyle best practices for minimizing the risk that they'll wake up next full moon with some part of them smashed off and needing to be regrown, or worse still not waking up next full moon at all. And the first of these questions has to do with what type of stone they'd want to make themselves out of.
See, it's not just some generic gray stone material that they turn into. When the full moon ends, gargoyles must be touching and concentrating on an object made out of a type of rock, be it a statue pedestal, a castle wall, or a cave, and their bodies will transform into whatever type of rock they were touching at the time, be that soapstone, basalt or marble. While they will be more durable than normal rock of this variety, these are just magical enhancements upon the pre-existing qualities the rock they turned into had, so the material they use as their base still has a significant impact on things like their weight and resistance to being smashed by hostile forces. And thanks to the advancements of modern society (say the late 90s), modern gargoyles find themselves having access to just about any type of stone in existence with which to use as their statue base. It's just a question of picking the right one for the job.
What material that meets the definition of "rock" would be the ideal material for gargoyles to turn into so as to minimize the risk of sustaining accidental or deliberate damage during the month in which they are otherwise helpless?