Howsabout this:
The majority of meteor strikes are just iron and nickel and junk like that, but large meteors can be coated in
iridium laced iron, or have iridium cores. Maybe one in a thousand has a single square of adamantium ore in it for no explained reason.
The tech level doesn't allow for dwarves to work pure iridium, with any degree of success (though perhaps a major skill penalty would be enough), but alloying iridium with iron, and forging it into steel would yield a very heavy, brittle metal, that is highly resistant to corrosion (it is totally immune to acids, metals and alki, but can be corroded by some molten salts).
Let's say that for every unit of iridium laced ore processed, there is a 10% chance of getting a bar of iridium iron, instead of a regular iron bar. That's probably way too generous, but we need the ore to be worth digging up.
To make meteors a bit more exciting, I propose that some of them be made of something altogether alien. I'll borrow the name from one of the old Tacticus games, and call it Unobtainium. It's a good name, with a rich history, and I defy you to better. To emphasise that this is something totally outside our experience, the stats on this metal could be randomized (within constraints, if it's boiling point is too low, it wouldn't have survived the fall, etc.).
--OR!
Unobtainium could be found only in the form of native unobtainium, that yields two bars to a mined square. Unfortunately that's because it's the wrecked vehicle of some creature from beyond our existence. I'm thinking creatures so crazy random that you never know what to expect inside.
There could be an eldritch abomination(i.e. a giant hostile titan), or a race of peaceful, sentient beings in there.
This could even be an early inroad for random civs. Every planet get's a small pod of these random aliens dropped on the world, around the time that the other civs appear. If they don't die off in worldgen, then the player has to siege them, for that precious sphere of metal they came out of.
Blood. Randomize whether it's a regular titan, who will emerge and abandon the eggshell, a titan that will sleep inside until disturbed, or a small civilization of creatures.