Fantasy worlds are full of magical materials – some actually enchanted, others merely unusual. In 3.5 D&D we have a whole host of them including mithril, adamantine, and darkwood.
Usually, there are weapons and armor made with these materials that are lighter or stronger than they normally would be. Rarely, though, are the full ramifications of these materials for engineering recognized.
Some settings deal with these things better than others. Exalted is pretty good for this with a host of magitech, but even it only touches the surface. In Exalted, the Haslanti League mines feathersteel – a remarkably lightweight ore that they use in building airships. The old city of Chiaroscuro had buildings built of a strong, glass-like material. The five magical materials: Orichalcum, Moonsilver, Starmetal, Soulsteel, and Jade are all nearly unbreakable possessed of different affinities. While not a building material, firedust is a naturally-occuring substance similar to black powder.
What, though, could exist in a fantasy world with some basic magic and access to materials like mithril, adamantine, and darkwood? (I am deliberately ignoring the possibility that these materials might be fantastically rare and expensive.)
Starting out in the familiar realm of weapons, we have untouched possibilities: mithril (half the weight of steel, but just as strong) could be used to make oversized, but usable weapons. Want a longsword that feels like you’re wielding a shortsword or rapier? OK. How about a five foot long blade that’s usable one-handed? Leverage will get to be an issue eventually, but a lot can be done with counterbalancing. Adamantine (super-strong metal) makes weapon-breakers a really feasable possibility. Darkwood (half the wieght of wood, just as strong) might have some interesting applications for long polearm hafts. A darkwood quarterstaff would be light enough to have heavy iron butts on either end – essentially, a dual mace with extra leverage. Adamantine could be used to make finely-linked (jewelery-sized) chains that are stronger than those found on normal flails – these could have a host of uses in weaponry and elsewhere.
Armor is often a reflection of the materials available. While D&D posits lightweight mithril full plate armor, what about normal-weight, extra thick full plate made of mithril? What about leather armor with mithril inserts to add extra protection to vital areas?
Stepping away from armor and weapons, what would a boat made from darkwood look like? Could a boat be made of mithral?
What about skyscrapers?
Think about what we can do with materials like plastics, high-density ceramics, steel alloys, and even aluminum – things that weren’t existent or readily available 1000 years ago. Now, our current interests and needs are likely somewhat different from those of people living in a fantasy setting, but if you were to combine some of the fantasy materials above with spells like Wood Shape, Mending, Stone Shape (who needs concrete?), and Fabricate, I expect our engineering capabilities would be similar.