Are Clams worth empowering for?

[Tuidjy]

I do not like clams/fetishes/blood gems, but whenever it is not too much trouble, I
craft them. They are a damn useful thing to have on a scout sneaking along a
conquering army. And if the game looks like a long one, they're a good strategy.

If it were up to me, I would make the probability of each item generating a gem on
any particular turn be

numberOfProvincesOnMap / (5 * numberOfItemsOfThisType)

Rounded down to one, of course. I dislike putting a hard limit on items, as
this gives a very solid advantage to the first player to craft them.