The Natural Magic of Stones
For September's RPG Blog Carnival, host WobbleRocket at his eponymous blog chose the topic Magic Stones. Among possible examples, WobbleRocket gives the following: "In Dungeons and Dragon's Forgotten Realms setting, there’s an entire subset of magic related to unlocking the powers of real-world gemstones like tiger’s eye, hematite, and onyx." Of all the ways to approach the topic, this one I find the most interesting. If apothecaries study the natural powers of plants, why wouldn't alchemists and artisans in a fantasy setting study the natural powers of stones – which is to say, their naturally magical powers? Using regular stones rather than fantastic ones also gives the setting a touch of the real; not everyone cares about verisimilitude in their tabletop games, but I do, up to a point. It would improve the illusion of plausibility even further if the magical properties of the stones were based on the stones' real properties.
When I looked at the wiki page that WobbleRocket linked to, however, I was disappointed to see that few of the stones' magical powers seem based on their real properties. Let me therefore suggest some stones with interesting real properties on which we could base some magical ones.
The Crystal Ball, John William Waterhouse, 1902
Adamant
"Adamant" is an archaic word for "diamond"; you can see the etymological relation if you compare the two. Diamond is the hardest natural substance on earth and at one time "adamantine" meant "as hard as diamonds" or, perhaps, "made of diamonds." I like to remember that when reading lines like this one, from John Milton's Paradise Lost:
at last appeer
Hell bounds high reaching to the horrid Roof,
And thrice threefold the Gates; three fold were Brass,
Three Iron, three of Adamantine Rock,
Impenetrable, impal'd with circling fire,
Yet unconsum'd (Book II, line 643-648).
Are those gates' last three folds made of solid diamond?
I'm tempted to say adamantine armour and weapons in fantasy settings are made of pure diamond, formed somehow by artificery. This alteration would be visually splendid, a good touch for a high magic setting. But if you have a reason to keep adamantine a metal (maybe the sheer expense of a greatsword's volume in diamonds makes you blanche), you could say instead that adamantine is a steel alloy in which diamond dust has been mixed and treated by gemworkers to imbue the metal with the jewel's natural hardness and imperishability.
Autunite
Yellow-green autunite contains radioactive uranium. It also fluoresces under ultraviolet rays; in other words, when a particular kind of light falls on it, it produces a glow of its own. That's enough reason for me to imagine the mineral would have something to do with light. When someone casts a spell like 5th edition Dungeons & Dragons's light on an untreated autunite crystal, the spell lasts twice as long and illuminates a space twice as wide as it ordinarily would. Autunite that's had its more secret properties teased out by artificery will, when used as a spellcasting focus, prolong the duration of any illumination spell. Armour that a gemworker has studded with these stones will also return a portion of radiant damage back against attackers.
Lodestone
Naturally magnetized magnetite is also called lodestone and it was used in the first compasses. Lodestones also, of course, attract certain metals. Valuable enough just for use in navigational instruments, perhaps lodestones' natural magnetism is also useful to artificers. I imagine it would be hard to work with magnetite, but expert crafters might be able to make shields imbued with its properties. Such shields would draw iron and steel blades and points toward themselves, better protecting their wielders from these weapons.
Quartz and Topaz
Because of the arrangement of their particles, quartz and topaz are piezoelectric: when compressed, they release a small amount of electricity. (Furthermore, when an electric current is passed through them, they compress, but that's not my interest here.) Of the two, quartz is by far the more common and topaz the more expensive, valuable not just for its rarity but also its colour and lustre. The stones' piezoelectric nature suggests an affinity for lightning magic.1 Spells that deal lightning damage are stronger when cast with quartz or topaz foci, and weapons studded with either can, when properly made, deal an occasional jolt on a successful strike or parry. Note also that there are a few varieties of quartz known by other names: amethyst, citrine, and vermarine, which is also called prasiolite.
Sunstone
Iceland spar is likely the mineral known in the 13th and 14th century as "sunstone," used by sailors to navigate when the sky is overcast. It is almost completely transparent, but it splits light that passes through it in two, so that anything seen through it is doubled. The theory is that medieval navigators held the stone up and the doubling of the light allowed sailors to locate the sun behind the clouds, in a way I admit I don't quite understand.
Sunstone is therefore appropriate for tasks that involve knowing. Divination magic is more reliable when cast with properly treated sunstone; a crystal ball is therefore best made with Iceland spar, as are dice used for telling fortunes. Perhaps spellcasting foci made with the crystal, when its properties are brought out by artificers, also produce better simulacra and illusory doubles.
Is this why topaz causes shock damage when infused onto a wand in The Legend of Zelda: Tears of the Kingdom?↩