Bette reported it was very difficult to cut through the "vitrified rock." Gold is a very SOFT metal and I would think it would not have posed such a problem.
It is true that foil sheets (made or gold and other metals) are used on sattelites and space vehicles because of their strength and resiliancy (sp?) in free-fall, but Mars has gravity reasonably comparable to Earth and the substance in the walls cannot be refined and homogenous enough to simulate such sheet regardless.
Could that object be something in the diamond family? There is lots of carbon on Mars. The walls have an unnatural look to them as it is and could be manufactured from local resources. If the heat and pressure were sufficient, diamond-like lattices could have been produced (probably in large patches) that might be what has been found.