@RIOT Skin Team, A few simple questions about Gemstone skins.

Most skin tiers are pretty well defined these days, so we understand what makes a skin Legendary, Epic or whatever and it has been stated that often you decide what tier a skin is going to be based on what is required to fulfil the fantasy of the skin. From what I understand, a Mythic skin is effectively an exclusive epic skin, originally from gemstones in hextech crafting, but now we also have Prestige skins.

What I don't understand is how the decision is made for a certain skin to become a gemstone skin rather than a normal Epic buyable skin. I understand that all new Hextech skins have been gemstone skins (which makes sense considering it is Hextech crafting), but there are a bunch of exceptions like Dreadnova Darius, Lancer Zero Hecarim, and Soulstealer Vayne. So why was it decided that these would be gemstone skins?

I am just a little curious on what the decision making process with this is.