What is this rock doing?

So, I was in Guatemala for two weeks on a mission trip, and on the trip we all hiked up Pacaya Volcano.

It had erupted about three months ago. But, once we got to the top, we roasted marshmallows at the hotspots where all the lava rocks were. I decided to take a couple home with me but have no idea what’s happening.

They’re starting to crystallize or something and I’m really confused what’s happening.

At first they were red, but now they’re black and crystaly and I just want to know what’s happening.

Can the science side of fluther tell me what’s happening to the rocks?