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Seriously tho if you can accept Water and Ice as different, despite being the same substance, with the understanding this split is necessary/beneficial, then Rock and Ground should be same. They're just not AS different as Water and Ice, but the differences they do have are significant enough to warrant a split especially in an elemental animals RPG where the physical interactions of these things matters for Type effectiveness reasons.
Ground has sand and mud as sub-themes. Rock has fossils, coal and gemstones. They're fine seperate, they aren't all that similar really.