#celestial bronze

*Note: this refers specifically to metals that Greek and Roman demigods can use. I'm not including other pantheons*

*Most common in this sense refers to most commonly used by demigods*

So we, of course, have the canon Stygian Iron, Celestial Bronze, and Imperial Gold, but in addition, we also get Adamantine, Orichalum, Artemisian Silver, and Apolloin Gold.

Starting off with more facts about the basic metals (ones known in the series):

Celestial Bronze: most common mythical metal. Primarily used by Greek demigods, however anyone within the two pantheons can use a celestial bronze weapon. Can't kill mortals.

Imperial Gold: equally as common as Celestial Bronze, only "rarer" among demigods because Roman Gods are the ones who control the mining and usage of the metal, and these gods are stricter regarding privledge to use the metal. Primarily used by Romans, however anyone within the two pantheons can use a imperial gold weapon. Can't kill mortals.

Stygian Iron: third most common mythical metal. However, stygian iron can only be used by chthonic demigods, gods, and other underworldly creatures. If stygian iron is used by non-chthonic demigods, it weakens their life source and can take years from them. If used by a non-chthonic god, their powers are weakened for many years. Stygian iron cannot be used by mortals-- it kills them near instantaneously. Stygian iron can kill mortals.

Now into my own ideas:

Orichalum: tied with Stygian iron for third most common mythical metal. Mined in Atlantis, orichalum, like stygian iron, cannot be used by all beings. Instead, orichalum can only be used by aquatic gods, demigods, and creatures. In this sense, aquatic refers to both the ocean (Poseidon, Oceanus, the Nereids, etc), but also all other bodies of waters like lakes and rivers (river gods, nymphs, Naiads). Riptide is not actually made of celestial bronze, it is made of orichalum. It's effects on non-aquatics are similar to that of stygian iron. Orichalum cannot kill mortals. Sort of bronze-ish looking.

Apolloin Gold: the fifth most common metal. This can only come directly from Apollo or Helios, as the metal is quite literally ored from the sun. Technically Eos and Hemera can also give it out, but they have many fewer connections with mortals. As such, should it be one of the rarer metals? Yes. It's more common than it's parallel, however, because Apollo used to hand the metal out quite frequently-- to mortals who impressed him, his lovers, his children, and the metal just made it's way into other people's hands. Can be used by anyone. Cannot kill mortals.

Artemisian Silver: the sixth most common element-- or second rarest. Artemisian Silver can only come from Artemis, Selene, or Hecate. As Artemis is the main goddess of the moon at this point, artemisian silver most closely represents her other domains, and as such can only be used by girls and women, *except in cases of Hecate's boy children or those blessed by Artemis*. Artemisian silver can harm werewolves, which is where the silver werewolf stereotype came from, and why Reyna's pocket knife, although silver, did not work. Cannot kill mortals.

Adamantine: rarest of all. This metal is most closely guarded by the gods, and only a few known weapons remain in demigod weaponries. Bright, almost blinding white, the metal looks like a cross between diamonds and silver. Completely unbreakable, and cannot be bent or harmed in any way after being forged. The only demigods who can use adamantine are children of the Kronides and Aphrodite (eldest gods), although even many of these demigods have to dilute it to use the metal. It burns through other demigods and mortals until their hands are ash if they touch it. The full olympian council must decide to give any demigod an adamantine weapon. It can hurt mortals.

There's also, of course, Drakon bone, although that's not technically a metal. Drakon bone is the only well known bone weapon in the series, however, divine bone from any being can be forged into a weapon that can kill monsters. Has to come from a spoil of war, as the being will dissolved if killed, and as such, is never seen outside of Tartarus, with the exception of Annabeth's sword.