Peakstone is a dark, nearly black natural block found on Mountains.
To mine (take) Peakstone, you will have to equip a Stone Mining Cell or better.
Since Rambeaus will spawn on or close to Peakstone during the day and Arctic Mirus during the night, you can build artificial biomes / arenas from Peakstone to make these Creatures spawn - and occasionally Wood Treasure Chests in the night in darkness too.
Blizzard Chizzards will spawn on or near Snow that is often close to areas consisting of Peakstone, but if you build arenas from Peakstone without any Snow nearby, no Blizzard Chizzards are going to spawn, only Rambeaus and Arctic Mirus.