A deity's favored weapon is mostly flavor. Clerics can use whatever weapons they want, regardless of them being a favored weapon of their deity. There are a couple of mechanical effects for a deity's favored weapon though.

1. Clerics of the War domain receive Martial Weapon Proficiency and Weapon Focus for their deity's favored weapon if they do not already have it. This only applies to weapons that require Martial Weapon Proficiency. Simple weapons, such as a quarterstaff, are already in a Cleric's proficiency, and exotic weapons such as a claw bracer still require you take Exotic Weapon Proficiency. You still receive Weapon Focus as a bonus feat regardless of the nature of the weapon.

2. The spell spiritual weapon summons either your deity's favored weapon or a weapon with a spiritual connection to you. Spiritual weapons all do the same damage, but retain the threat range and critical multipliers of their shape.

3. The Clr spell weapon of the deity (Spell Compendium 237) grants the caster's weapon special abilities based on the deity the caster worships if the weapon the caster wields is the deity's favored weapon.

Favored weapons are used mostly as a roleplaying device. Warriors of a particular deity might be discouraged by their church from using weapons other than their deity's preferred weapon, for example. There is no mechanical penalty or benefit (other than the two listed) unless the DM creates one through storytelling.

A diamond is not a metal object

While David offers a great answer to making it work, I'm going to offer the other side of the Artisan's Blessed coin; I don't think this is the intended purpose. Magic has a component cost for a reason, and you're trying to abuse a class feature as a loophole to get around this.

You conduct an hour-long ritual that crafts a nonmagical item that must include some metal: a simple or martial weapon, a suit of armor, ten pieces of ammunition, a set of tools, or another metal object.

A diamond ring is not a weapon, a suit of armor, a set of tools or ammunition, so it must fall under 'another metal object'. If you are making a ring purely for the diamond contained within, you're not really making a metal object, are you? Once your DM allows you to create a cheap brass ring with an expensive 99.9 GP diamond in it, this feature essentially reads "you can create any mundane object costing less than 100 gp".

Need rope? "Yeah I'm making a grappling hook with 900 feet of rope, and then I'm cutting the hook off."

Need perfume? "Yeah I'm making a perfume bottle with perfume in it, with a metal stopper."

Need a chest? "Yeah I'm making a wooden chest with a metal lock on it."

While your DM obviously has the final say in all matters, and some DMs will allow this kind of thing, the feature says you can make a metal object, not "anything you want, with some cheap metal attached."

Want to store your diamonds? You can use your artisan's blessing feature to make rings worth no more than 100 GP out of metal with an opening, and then afterwards manually add an existing diamond to it.