They are worth what someone is willing to pay for them. Just like everything in a free market. And they are worth what he is willing to sell them for as the other side of that token. If they are worth 190 diamonds to him(10 dia ea), then he wont sell them for 50 diamonds. So they are worth more than 50 to him. and if no one is willing to pay more than 11 diamonds for them, they are technically worthless. Because you wont ever sell them. But, where did you get gravel? Shouldn't we wonder that?
Gravel circulates the economy. And while it would be absolutely wonderful if the barter system worked like that, it doesn't. Gravel's worth 1-2 diamonds a piece. It would be foolish of him to sell all of it for under 19 diamonds, but he also can't expect anyone at all to buy it for over 38.
