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.