Should I be forced to sell my rock, because I'm getting a $500M tax bill from the government because some weird little cult said they would pay $1B for it - regardless of whether I want to sell my rock?
If you don't want to sell your rock, then set up a family trust to hold the rock with the provision that it can never be sold, then the rock will have effectively zero taxable value and you can keep it in the family forever.
If you don't want to do that because you might want to sell it some day, then it's not really a family heirloom, it's an investment.
Or, put the rock in a trust, tell the cult that they can take ownership in 100 years and have access to it once a year for ceremonies if they'll pay you $100M today.
If you don't want to sell your rock, then set up a family trust to hold the rock with the provision that it can never be sold, then the rock will have effectively zero taxable value and you can keep it in the family forever.
If you don't want to do that because you might want to sell it some day, then it's not really a family heirloom, it's an investment.
Or, put the rock in a trust, tell the cult that they can take ownership in 100 years and have access to it once a year for ceremonies if they'll pay you $100M today.