They are also all made of granite when in 1.7.10 I believe they only spawn with Rhyolite. Wiki has no info so I can't confirm it. Allowing a random Igneous rock to spawn would be a nice touch if possible. Seems to change already, oops.

Two fer one!

Fissures are able to spawn with non-hot water, i.e. lava or fresh water. However this is already noted in https://github.com/TerraFirmaCraft/TerraFirmaCraft/projects/2#card-17673269