[Lazy_Perfectionist]

From what I recall, barring special maps... Though what I recall is quite limited, and hardly the entire deal...

Terrain has certain modifiers on probability, and sites that can be generated. We'll assume 40%, however.

When the game is started, each province (capitals excepted) gets four separate rolls for sites. For each site that is found, it rolls for what kind of site is there. If that roll produces a rare site, then it rerolls to see if it keeps that site. If it produces a unique site, and that site is already taken, it selects another.

Wastelands have a higher chance of having sites, while farmlands have a lower chance. Once its figured out whether a province contains a site, the probability of a province containing a site of a particular path is determined by the number of possible sites. That is, you'll never find blood underwater, death is unlikely, and water common, simply because of available selection.

A site can only be of one path. It can generate gems of more than one path, but it still belongs to only one path. There are water sites that produce fire gems, and fire gems only. You can get a clue by looking at the icon, or loading up Edi's database and sorting the sites sheet by type.