Pokémon belonging to the Mineral Egg Group are inorganic as they consist of rock, stone, crystal, or metal. Their appearance is mostly natural, resembling stone formations, but some appear to be human-made objects infused with life essences, either on purpose or accidentally. Remember that while most of them are genderless, Ditto is still able to produce offspring with all of them.
It is to note, that several theories circulate the web that members of this Egg Group might showcase how life-based on silicone could look. For reference, every organic lifeform here on earth is carbon-based, and silicone is assumed to be another potential stable and life-enabling component, making these creatures potentially very realistic.