User blog comment:Plexioth- xD/Monster Hunter 3 Ultimate/@comment-3095304-20130321021736/@comment-130804-20130321122428

The area with the ice plateau tends to give the best chance. A blue rock ALWAYS spawns there.

General rule of thumb: blue = rare material rocks.

The volcano also has a third type or rock, that I dub the Gem Rock. It normally gives jewels, charms, and ultra-rare materials the most often