I'm currently designing a sapient species for a science fantasy setting that's supposed to be a herbivorous, physically resilient, and bulky sort. They currently bear some resemblance to boars or minotaurs for comparison, but that's currently only the 'foundation' of their design. One thing that characterizes a lot of the species in the setting are their distinct evolutionary traits that inform their character designs, such as one species being able to cultivate mushrooms on themselves, or another with colour-changing sail-like ears for social communication.
I've been experimenting with the concept of the current species being able to grow mineral deposits on themselves as both natural protection and as a form of blunt weaponry, but I'm unsure about the specifics of how to justify this trait without resorting to an excessive amount of handwaving. Of course, the setting is still a science-fantasy blend, so as long as it can be justified to a decent enough degree without having to invent an excessively convoluted setup, mineral or environment, I'll be happy with it.
There's a few ways that I could go about it - such as making it so that the horn itself is actually still just a normal horn (if a little rocky looking), and the crystals grow on top, I could have the horn itself be made entirely out of crystal that's grown on the body somehow, or have it be part of the species itself. I'm reluctant to do something more exotic like, "this species' bones are made out of rock!" mostly because I wouldn't have all of the requisite knowledge to continually justify it or integrate it into the setting or the species' culture as smoothly as I would like. Gastroliths turned up in my research as well, and while most species obviously can't integrate stone and minerals in their body from digestion, that's a potential 'in' for justifying how it works biologically.
There's a few other things that come to mind on this topic, like the formation of Stalagmites and Stalactites, but the problem with those is that they would take an excessive amount of time, nor do they really work on biological species, to the extent in which an entire horn could form. Things that could form 'actual' gemstones like rubies are also highly improbable or impossible without significant intervention as those are tied to geological processes as a whole. Ideally, I'd like for the crystals to happen 'somewhat quickly', and to be part of the species' natural evolution in their original environment instead of being an artificial addition in modern society. It would also be nice if the formed crystals were a little sparkly or glittery, and of course, the formed protrusions would have to be resilient enough to actually serve their purpose as an offensive and defensive tool in the species' arsenal.
I'd appreciate your suggestions and ideas on this, as mineralogy is a field I'm inexperienced in, and searching for more information on fictional takes on mineral/biological combinations has been rather difficult.