Membership Tiers and Rocks

Bronze, Silver, Gold, Platinum, Diamond-- whether it's airport privileges or membership tiers, more valuable materials get higher status. My question is what the next level up will be once 'Diamond' memberships become too common. A search of the internet suggested 'moon rock' as more valuable than diamond, but it seems that moon rock is valued at about $3.50 per milligram while diamonds can sell for as much as $37.50 per milligram. With that in mind, my guess is that membership tier makers will stop beating around the bush and name the highest tier the 'Money' membership. Gold, Platinum, Diamond, Money. How much money? If you have to ask, you can't afford it.