After spiritbonding about ~200 pieces of gear, I think it's safe to say that all materia is 100% random.

It really doesn't matter which piece of gear it is or what slot it's in, the chance of obtaining any certain materia is the same for all gear across the board. The only difference is tiers, which seem to be related to the gear level.

Attribute materia is by far the most common, while Healer's Hand is likely the rarest of all (I've never made a single one).