Right inside AQ40 are 4 giants :O. If you kill one of them you will be able to mine a large obsidan chunk. Kill three of the four mobs and run out of the instance and there will be 3 mines that will appear. Just be sure that you don't kill all 4 mobs and you will be able to repeat this as many times as you want. The large obsidan chunks sell for 200g to 300 gold on the AH, on my server. So, you can make awsome amounts of money by using this method, happy mining!