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I would say that makes that particular rock an egg. At least for me, once an animal hatches from it, it's not a rock that hatched an animal, it's an egg that looks like a rock.
In the movie Gamera vs. Barugon, some guys take a large opal from a cave. The opal is rounded and smooth due to the All-Natural Gem Polish trope. Later, a small animal hatches from the opal (actually an egg) and quickly grows into a Kaiju. At least in the English dub I watched, nobody refers to it as an opal anymore, but they never mention whether the eggshell remains would be valuable as a gem-quality opal because it becomes unimportant to the story. The movie proceeds as if it was a monster's egg that was mistakenly identified as an opal.
Edited by FerrousFaucetIf it hatches, it's an egg. If the game claims it's totally not an egg, honest, the game is engaging in Blatant Lies (likely for the funnies).
Quite possibly, but I'm afraid we don't have such a trope. Hidden in Plain Sight ?

A creature hatches from something that is not an egg.
In Fargo's Mod, it’s said that the Seeker of Treasures pet (a baby version of the Champion of Terra boss) hatches from a rock in the “Mostly Ordinary Rock” item tooltip:
I don’t thing anything in An Eggcellent Index applies here, as the eggs in those tropes are relatively “normal” eggs, not something like this.