Alloy is a Secondary gene. You can get it through the gem marketplace.
It costs 1200 gems to buy.
I don't have strong feelings one way or the other about Alloy. It is an extremely shiny gene so I, being a child at heart, like it for that reason. The thing is that it doesn't impress me too much on its own. This might indicate that it's a gene that's awesome if paired with the right genes, colors, and breed.
If you have any examples of dragons that work or don't work with Alloy, please post them in the comments. I might do something with them later.