in addition to the old Gygax root material predating later D&D editions, there's also deity based totems, like all the holy symbol components of various gods, and then the many tropes found in the flavor text of spells and monsters known for being associated with alignments, or planes of existence famous for alignments.
For example: When I say Fire and Brimstone, you think what?
Feathers, Clouds, Golden Halos?
So things like Sulfur are associated with 9 hells and things like white feathers are associated with upper planes and good. Water is considered calm and turbulent, and amber is associated with electricity and lightning. But the elemental planes are traditionally neutral.
Mithril is associated with magic, dwarves, and elves, and Adamantite is associated with indestructibility or immortality, and occassionally dwarves. black ooze or dark colored ichor is usually associated with evil, as are spider webs, especially in D&D with Lolth.
I'd avoid putting silver and law in the same box. Silver is associated with the moon, and the Sun and moon are traditionally rivals, even as embodied deities in the various pantheons, and Sun is almost always some ruling lawful or lawful good figure, while Moon is generally thought as the opposite, disobedient, or rival. Sometimes Sun is male and moon is female, and you might have Lawful Neutral Sun and Chaotic Neutral Moon. Moon is especially associated with magic, but also silver. The Amaterasu goddess is associated with the Sun so it's not entirely true to assign masculine to the sun.
Red Gold, in D&D was associated with Greed (one of the older campaigns), which is normally a chaotic neutral thing, and anyone near it got gold fever of a sorts. Ash/Dust from cremation is almost always associated with death, and death is traditionally associated with neutral evil, like the demilich from Planescape that led the Dustmen.
Green Steel specifically lawful evil, because its associated with the weapons and armor forged by devils in the lawful evil 9 hells.
Quicksilver is associated with transmutation and change, while oil and tentacles and swirls are associated with chaos, so gemstones with swirling rainbows would work.
A unicorn horn or aasimon feather will always be associated with good, but these aren't metals.
Iron was associated with Mars, and thus War in Alchemy. Gold with Sun, Sunday, and Holy or Lawful events. Volo's Guide to all things Magical is an old book with a whole section on magic related to gemstones. Since many spells have a kind of alignment to them, that might help. Druids for example used to all be true neutral, so a plant spell or plant gemstone or nature related item would also be true neutral.
Best of luck!