Rocksters and Pebbles are both spawning on beaches - on Sand close to Water. They are peaceful creatures. The aggressive Night Rocksters will only spawn in darkness; not only on Beaches and on Sand, but also in caves and on the Fossil layer underground. Night Rocksters cannot prevail in sunlight, but will burst into blue flames and burn away. To make 3 Pebble's Pebble into 1 Rockster Rock each, put them into the Forge and add some Fuel. Rockster Rocks are needed as crafting materials for Leather armor.