Note that materia spirit bonding rates are based on item quality and number of melds that item currently has, leading to hq crafted penta-melded items spirit bond the quickest.

What materia is extracted is based on the stats and level of the item it's extracted from. I do not know if stats that are melded to it also have a chance of being extracted. Meaning if you have a level 15-20 combat chest piece with say 5 crafting/gathering materia melded to it you will likely get a combat-based materia when it reaches 100% and you extract it. I don't recall ever getting a crafting or gathering materia from a combat armor piece, but that's not to say that it might not happen, it might just be a really really low chance. Note that this requires that the combat gear is wearable by all jobs so you can wear it while being a crafter.

I talk about using crafting/gathering materia with a combat item, mainly because those are generally cheaper to purchase, and you will likely destroy some in the process of penta-melding pieces.

I am unsure if the quality of the item you craft affects the spirit bonding rate, but it's level in compared to the items equipped is. When you craft items at or above the level of the gear you are wearing, their individual spirit bonding rates goes up a larger amount.

Adding boosts to spirit bonding rates also helps, but the items do have a cap as to how much their spirit bonding can increase per craft. The amount also is partially random. Essentially adding these increases the minimum amount of spirit bonding you will get per craft in each item. There are items that can boost this rate as well, such as the grand company ear slot item Lily and the Serpent Earrings (Gridania gc, haven't checked the other two but I'd guess they have a version as well)

Now successfully crafting an item is based on your total stats, so if you use high level crafting main and off hand, you likely won't need any additional stats from other equipment, so you can put combat pieces there instead. What you will want to have though is enough CP to cover the 250 of Trained Eye and enough CP for the Synthesis steps to complete it.

This all adds up to crafting items that are above the level of the gear in combat gear of the level of materia you are looking to get.

In the end this is what I mean for creating materia I's...



Now aside from crafting/spirit bonding, there is another way. That is using Mutamix's machine to mash 5 unwanted materia into one "random" new materia.

Personally, what I would love to see them add is a reverse of this process, where you take a higher level materia and smash it into random smaller ones.