Since this word consists of a kanji with hiragana attached, you can bet that it will use the kun'yomi reading. You didn't learn that reading with this kanji, so here's a mnemonic to help you:
Imagine something rolling down a hill. Inside is a very important uranium core (ころ) that you need to keep spinning. That's why they have it rolling down hill after hill, because they don't have anything else to keep it spinning like that, preventing it from exploding. This core is very important!
If you have trouble remembering the hiragana ending, just imagine a girl (がる) rolling down the hill too.
Your cell phone was left somewhere around here.
Darin discovered the dead body of a blowfish, lying in front of the Tofugu office.
A rolling stone? Do you mean the English rock band The Rolling Stones?