Gold is irreplaceable: Almost half of the gold sold is used in electronics and medicine. It's not just about jewels.
Gold is unique: it has unique properties.
Gold is scarce. You could fit all the gold in the world on a tennis field.
Gold is not irreplaceable it's price has no correlation to it's industrial nor decorative uses.
The scarcity is only a temporary condition.
Probably in 100 years we'll start mining asteroids and then gold will no longer be so scarce.