Earlier quoted context omitted.
> Both are in fixed quantity so none is more rare than other. There is 80% more gold aboveground today than there was in 1980.
Gold exploration had been going on for thousands of years which means every new piece of gold is being discovered at much higher cost. So while supply increases, prices don't go down.
Also keep in mind that a technological breakthrough could produce a sudden glut in gold supply, as we have seen recently with oil and gas fracking.
The main reason the price has gone up is that the world population is in aggregate far richer than in 1980. What's more, Indians and Chinese (who are culturally inclined to invest in gold) have seen their wealth grow particularly rapidly, so the proportion of world assets stored in gold has likely increased.