So, it really hasn't come up in this game, or even in these forums, but I am a bit of a rockhound, and a collector of mineral samples. At many times, I have found myself staring at some of my pieces that have interesting shapes, or uncommon variances, pondering over them for swaths of time, just looking at the tiny, tiny details of them, and, on my way to my college today, I realized that isn't that the very thing that curiosities are supposed to be?
My idea, as it is, is to implement various types of minerals to the game that can act as curiosities, much as the currently implemented cat gold is, perhaps functioning almost identically (save perhaps that maybe they can also be smelted, perhaps directly into nuggets of their given metal: these are just supposed to be just striking examples of their mineral species): being random "drops" when mining or digging in different ares, dependant on what kind of rocks or ores are in the area, or even no rocks at all per se, given that minerals can form anywhere where atoms are free to move around and into the crystalline matrix of the mineral in question.
For some examples:
Quartz
Found: any rock or ore
Has a vertually endless veriety.
Magnetite
Found in: iron ore
Its main attraction is that it's magnetic.
Malachite
Found in: copper ore
Vibrant green banding.
Cassiterite
Found in: tin ore
(really it looks almost identical to magnetite, consisting of black orthohedrons, but I'm adding it for completion's sake)
Horn silver
Found in: silver ore
Dull lustre, and shaped like a cow horn, hence the name.
Gold leaves
Found in: gold ore
Native gold has a propensity towards forming into shapes that can, and do, look like plants, this is a very obvious example.
Goethite
Found in: swamps
Called "bog iron" for a very good reason.