anyway what does the rock which sells for 20 million do?

Unobtanium proved to be the most baffling of scientific discoveries in the area of superconductors as it had a extremely strong magnetic field, reversing prior knowledge that all superconductors repel magnetic fields. Furthermore, unlike the fragile crystals of human-created superconducting compounds, the substance found on Pandora was a stable quasicrystal with its atoms arranged in a never-repeating but orderly pattern with fivefold symmetry. This structure was not only structurally rugged but also has mircoscopic voids in the quasicrytalline structure that contain the magnetic flux lines. Unobtanium has a unique magnetic field and properties of superconductivity, causing it to levitate. On Pandora, the magnetic effect causes huge outcroppings of Unobtainium to rip loose from the surface and float in the magnetic vortexes. These huge islands, named Hallelujah Mountains by Earth's explorers, are called Thundering Rocks by the Na'vi, who hold them sacred. The unique magnetic properties of Unobtainium are used to contain and direct the energy of the Matter-Antimatter annihilation which propels ships like ISV Venture Star. Without Unobtanium, interstellar commerce on this scale would not be possible. Unobtanium is not only the key to Earth’s energy needs in the 22nd century, but it is the enabler of interstellar travel and the establishment of a truly spacefaring civilization. Making a feed back loop, the more Unobtainium is mined, the more ships can be built, and more mining equipment can be sent to Pandora .