I'm making an amulet, what craft skill am I using?
Magic Item Creation: "To create magic items, spellcasters use special feats which allow them to invest time and money in an item's creation. At the end of this process, the spellcaster must make a single skill check (usually Spellcraft, but sometimes another skill) to finish the item. If an item type has multiple possible skills, you choose which skill to make the check with."
Creating Wondrous Items - "Skill Used In Creation: Spellcraft or an applicable Craft or Profession skill check."
There are various item types in the wondrous items, rings etc... Do I need craft ranks for each individually?
The Craft skill can be used untrained. Ranks in Craft (jewelry) would help if you're making jewelry, Craft (armor) would help when crafting armor, etc.
Or, if you're not actually creating an amulet from scratch, but enchanting one you bought, you could just use Spellcraft.
Profession - jeweller would cover all the basic jewelry. What the DC is would be highly unusual...its no harder to create a ring with a garnet inset then a ruby, but the 50 gp vs 1000 gp difference would mean it would take months to make the ruby item.
Using spellcraft without it being a class skill and having caster levels is going to generate huge penalties on making things. Possible, but probably not realistically useful.
==Aelryinth
Using spellcraft without it being a class skill and having caster levels is going to generate huge penalties on making things. Possible, but probably not realistically useful.
Yes, if you don't have a caster level, you'll want ranks in craft or profession in order to use Master Craftsman.
Since most people making magic items are spellcasters, and spellcasters usually have ranks in Spellcraft anyway, it's frequently going to be better or at least the same as a Craft skill.