Sure it's the rarity, but also the lack of corrosion. If your coins maintain the same weight for 100 years that's good, because the value can be measured by we

Sure it's the rarity, but also the lack of corrosion. If your coins maintain the same weight for 100 years that's good, because the value can be measured by weight more reliably. Contrast that with iron that degrades, or copper that actually increases in weight as it goes bad... now how do you weigh out a coin?