I assume it has to do with your levels. I'm 99 mining, 99 smithing - have only played solo games, and am a measely level-10 dungeoneering. I get multiple gorgonite and promethium rocks to mine EVERY dungeon I play so I make the best stuff I can each time. You need to team with a high miner/smither, play max difficulty, and have them make you a promethium spear. In my experience, they're not rare at all since I make 'em myself. :)