Gold is special because it is soft and dense, like lead, and as such is easy to work. It is very nonreactive, and thus doesn't rust, and isn't toxic (why it could be used to fill/replace teeth).
Lead again is soft and dense, which is why it can block radiation and x-rays so effectively.
Silver is chemically similar to gold, by its placement in the periodic table, though it is much lighter and does tarnish. Mythologically related to the moon, possibly through Greek myths of Artemis among others.
If my memory of LOTR serves me correctly, Mithril is truesilver, found deep beneath the earth, and has the fictional qualities of being stronger, tougher, etc than even steel, at the same time as being ligher and more flexible than steel.
Crystal is WoTC's odd invention, is supposedly psionically active, and counts as a metal only for rules purposes. It is most likely psionically shaped and treated to have similar properites to steel.
And adamantine is a mineral, a form of corundum (rubies are the red version of this gem) which is second only to diamonds in its hardness. It is also the name of a black colored enamel used on iron, created as a replacement for ivory. It is also in the LOTR, as I believe the gates of Bara-dur were made of adamant, which was supposed to be unbelievably hard. Adamantium is the unbreakable metal made famous by wolverine. A google search can give more info.
