I'm not an accountant or tax lawyer (in fact, I'm not any kind of lawyer). My layman's understanding is that value -- from goods and services -- is taxed when it moves between legal entities, be those people, estates, or corporations. This is not a prescriptive legal framework as far as I know, but is a descriptive framework which I have observed and which makes sense to me morally.
You paid income taxes on the money when you earned it because it left your employer's pocket and went into yours: the ownership of the value (money) has moved. You paid sales tax when you bought it because you exchanged money for the ring: the ownership of value (money, and a ring) has moved. And you pay an estate tax on it when it transfers from your estate to your children because, you guessed it, the ownership of value has moved.
You paid income taxes on the money when you earned it because it left your employer's pocket and went into yours: the ownership of the value (money) has moved. You paid sales tax when you bought it because you exchanged money for the ring: the ownership of value (money, and a ring) has moved. And you pay an estate tax on it when it transfers from your estate to your children because, you guessed it, the ownership of value has moved.