Spirit binding.

[Anova]

Which items you want to sb depends on what is selling. Generally though, spiritbound items will turn in to materia that increases one of the stats on the item. So, a ring that gives +ACC, +DET, and +Crit would turn in to an ACC, DET, or Crit materia. Whereas a ring that gives +CP is likely to make CP materia. Of course, it also has a chance to become an elemental materia, but you will not get a DET materia unless that stat is on the item.

If you are looking for DET and Crit, you will want Zircon and Spinel accessories, and DoW gear.

[subteraneanbird]

snip

A lot of this information is incorrect.

Here is what we know:

DoW/DoM gear has a chance of making almost any of the DoW/DoM materia. This includes the relative duds of elemental and main stat materia. The one exception is an item won't give a main stat it doesn't have on it. But all secondaries are fair game. There seems to be (but actually proving it is incredibly difficult) a slight weight towards stats on the item IE something with Det is more likely to give Det materia. It IS possible to get stats not on an item though. I have gotten Det materia from a Red Coral Necklace (INT, MND, ACC) and all sorts of stuff.

Crafting and Gathering gear is different. They will NEVER give elemental or main stat materia. They can only ever give the crafting or gathering stats (or both for pieces that have both). This makes them much more consistent because you are only picking from 3 options vs 10+. The one exception is Aetheryte Rings. The increased spiritbond stat is considered a "DoW/DoM" stat so they have all the risks that those have.