Decided a long time ago to see if any of the Evolution Stones were based on real-world minerals. Turns out, every single one of them is! Not really sure who's responsible (though I personally suspect Masuda, since he's the one that travels all over the place and gets 'inspiration' for the new games from it), but here's what I got for each of them:

Fire Stone = Oregon Sunstone

"It has a fiery orange heart."

Yeah, yeah, the Fire Stone is IRL a sunstone (and the Sun Stone isn't). Seriously, though, it matches - and not only that, there's a historical reason this one got picked. In 1987, a few years before Red and Blue began development in 1992, the Oregon Sunstone was designated the official state gemstone of Oregon after its discovery in 1980 - and its unique optical properties (the red bit in the center) made pretty much every gem hunter and mineralogist in the world take notice. While sprite artwork of the Fire Stone wasn't released until Ruby and Sapphire in 2001, the design was already relatively close to its final look as of 1998 (EP040, 'The Battling Eevee Brothers').

Water Stone = Iolite (aka Cordierite)

"It is the clear blue of a deep pool."

Known as 'water sapphire' for its deep blue-to-purple color, iolite has the unusual property of trichroism - depending on the viewing angle of an individual stone, it can display up to three different colors, changing when rotated. The first and most well-known is of course the deep purple-blue, while the second is mostly clear with a tinge of yellow - but the third is the clear with a slight tint of blue of a pool of deep water, much as in the description for the stone.

Leaf Stone = Leaf fossil

I get the strangest feeling that this one is self-explanatory... Regardless, the green color isn't too unusual - rocks containing small amounts of chlorine or iron can do that. Incidentally, there's a formation in Colorado known as the Green River formation that frequently has greenish-colored plant fossils.

Thunder Stone = Cat's-eye elbaite tourmaline

"It has a distinct thunderbolt pattern."

This one is actually well-known to Japanese audiences, as tourmaline is frequently associated with electricity and is known as 'the lightning stone' (deriving from its name in Japanese, 電気石, which can also read as 'lightning jewel'). Chargestone Cave in Unova uses the same pun. The cat's-eye effect is the result of hollow tubes within the gem that reflect a band of light across the surface - in more translucent stones, this ends up looking like a yellowish thunderbolt (though usually a more natural-looking one than the cartoonish bolt in the official artwork).

Sun Stone = Aragonite nodule

"It burns as red as the evening sun."

These are surprisingly popular at rock and gem shops and shows, since they look so weird. The crystal shape comes from the individual hexagonal columns of aragonite twinning hundreds of times during formation, eventually ending up as a 'ball' of crystals.

Moon Stone = Nickel-iron meteorite

"It is dark like the night sky."

This one's probably the most obvious of the Gen I assignments, since it's openly confirmed in descriptions about Mt. Moon across various games. The shallow imprints across the surface are called regmaglypts, and are a characteristic of metallic meteorites - on the way down through the atmosphere, the blasting effect of all that air resistance affects different parts of the surface at different rates, resulting in the indentation-like features.