This working with timeline things is nice if you want a reoccoring timeline, since the wizard will be alive at some point and dead at other points, but it leads to nasty paradoxes where a younger old wizard is deaded by the PCs who already deaded an older old wizard a few adventures back, so if he was already dead then, how could they kill him then and blah blah blah, ~Head Explodes~
So, I would leave the whole time traveling stuff to the gods and thier curse, so that if anything gets squirlly, the gods can just look at the PCs and say "Deal with it.
" and the PCs got to.
Moving on ...
What are they supposed to do with the Midium? Make shelters out of it? Trap the Acid Rain Worms that sound increadibly familiar to Thread from Anne McAffrey's Pern? What?
Also, What have you been doing in between games? Obviously all the events didn't happen in one session. Use the down time to plan out what to do in the next couple of games. Gives you a chance to stay a step or two ahead of the players, useful when you're making things up on the fly, which it kind of sounds like you're doing.
Moving on ... Some more ...
Making the bad guys vunerable to this metal sounds like a bad idea, cause no one knows where to find it. What you need is a direction to give your players and a bad guy to whoop on. Easiest way to do that is a couple of bad trips.
1st bad trip - Rip off Indian Jones and the Temple of Doom
Remember those stones they don't know they need? Time to clue them in. Have a group of canibalistic savages timeshift into thier way. The Pcs or someone they care about (hireling or attractive NPC maiden kinda thing) get captured by the cannibals and dragged off to thier Temple-in-the-Caves. Give them some gruesome scenery, gory skulls and human leather drums, cannibals gnawing the flesh off a BBQ'ed hand, that kind of thing. Now, give them some foreshadowing. Let them see some other captives, not the one they're rescuing, get lead to the cannibals idol
Remember Fish Mother? Ya, guess who the idol is carved like. But, add three hollows carved into her chest or somewhere convinient. In one of these hollows is one of the stones they need to open the portal.
Start stetting up the PCs escape. They get to see the stone in use. The cannibals have drawn a line in the stone floor. They prod a captive across the line and if he gets incinerated by a lance of fire, his people are deemed food and the cannibals eat the rest. While the PCs watch the cannibals prod a guy with a stone spear, the guy spins around and tries to drag the cannibal in, but the cannibal merely shoves the guy and his spear across the line and fwhoosh, all that left is a some smoke and a stone spearhead
on the other side of the line. Remember that, it'll be important later. Have them check to see if they notice it, and the one that does gets to be the sucker.
Time to give the PCs a distraction so they can escape with a clue. Have the stone act funny. When they start prodding the lucky PC (the sucker)across, the stone DOESN'T fry him. Instead, it glows and a ring of fire springs up on the line where the cannibals drew in the line. The cannibals don't have a clue what to do, mill around and the priest tries to tell them he's still in charge but no one listens, and all this chaos gives the PCs a chance to cut thier bonds with the stone spearhead and then pick up discarded weapons and such, and make thier getaway, but not before the person(s) thier rescuing makes them go after the stone to bring with them.
Bad Trip 2 - Enter the Dragon.
You need a bad guy. What bad guy is better than a Dragon? Ok, a demon, but we'll get to that.
They go through another Quantum Leap-esque time shift and they're Back to the Future. (No, I will NOT run out of bad cliches to rip off) Let them be in some post-apocolyptic urban wasteland. So they're waiting for Neo, but sorry, Neo is busy right now. They get to see a human riding some massive beast be chased down by robots and deaded. But while watching the show, they notice the robots are made out or MIDIUM!
Woot, as they say.
While the machines are busy hacking and sawing, some refugees spot the PCs and lead them to saftey from obvious slaughter into the underground. There they learn some info. EVERYONE here knows about Midium. The reason no one knows where any Midium is is because these machines have been gobbling it all up as time shifts around them. They're all run by one machine, called Big Blue. (Yes, I am horrible) Big Blue is a machine that has styled itself into a dragon. It's all made from Midium, like the other machines, but rather than running off electricity or steam or what you decided to have them run on, Big Blue runs on magic. Specifically, Magic coming from the blue stone in it's chest. (Hint hint, its a second stone)
The refugees haven't had any weapons in ages, all the technology to make chemicals or anything else has been destroyed. They're stuck at the same level as the PCs, but fighting robots. The guy that got deaded was a scout, and they managed to get something off him before he died. They think they can get the stone off the dragon, and end the rule of machines, if they can blind the dragon. They need the PCs to take care of that and then they will pry out the stone and the source of Big Blue's power. Pc's need to come up with a plan and do it, then they'll have two stones and alot of Midium. Yay them.
Bad Trip 3 - Hellboy anyone?
Time to get to the demon. New timeshift and they wake up in the bad collision of two ages: A Nazi in King Arthurs court. Highly discipled and oranized 'Imperial' troops are wiping out midevil scirmishers. The 'Imperials' big advantage: They have a demon wizard with them. He's got the third stone around his neck and uses it's power to make the shields and weapons of the 'Imperials' unbreakable as the bones of the earth.
Notice that? First stone is fire, second is water. Now earth.
Let the PCs at least wake up on the side of the good guys, the losers, and fall back in retreat when the city walls are all simultaneously reduced to rubble as the demon uses the earth stone to crumble them. The losers can have a scryer who recognises the stones the PCs carry and maybe the PCs and helps them figure out a way to use the two stones against the one. Maybe the earth stone doesn't work in the rain and the PCs can make the Fire and Water stones act together to remove his power. The losers can beat back the 'Imperials' since they can smash thier shields now and the PCs are left to fight the demon by themselves. If they don't seek him out, he will seek them out.
3 stones in hand, time to get the portal.
They've probably been tring to make thier way back to the Fishmother by now, if not, give them some subtle kicks in the rear. Sea creature ghosts haunting them, they wake up with dead fish in thier shoes, that kinda of subtlety.
So they get back to the island, and there's Mom, and so they go ahead and react like you've conditioned them to: Attack the thing that's bigger than us!
But, in true I'm-ripping-stuff-off fashion, they are stopped. By an army of the dead! Yup, all those sea creatures are back, glowing green and the PCs can do a thing to them or move through them. Don't they feel powerful. Ha Ha.
Let the sea creatures be gaurdians of the portal to the gods, and only ones judged worthy may enter. So now the various PCs have to do the hardest thing of thier quest. Apologise for deading all the fish guys. After they do, Mom will take the stones and use them to open the door. There, the gods can tell them about the cursed continent and tell the one with the amulet thanks and shower them with gifts, then drop them back amoung the acid rain worms and let them fix that problem.
There. I've pretty much handed you your campaign on a plate. Go.