The title tells us the speaker is a rock, but the feelings sound very human. The rock says it has been ignored, forgotten, mistreated, and even used, and that the people around it never understood or valued what it really is. The word "underestimate" stands out, because it suggests the rock was judged by its surface and written off.
The turning point comes in the middle, where the speaker says that beneath its "diverse colors" are "beautiful gems / Waiting to be found." Nobody looked closely enough to discover them. That makes the poem feel like a quiet comment on how people judge each other by appearances, and how much worth can stay hidden when no one bothers to look deeper.
The mood is sad and a little wistful, but not bitter. The last line, "It's quite a shame / They never gave me a chance," sounds more disappointed than angry. The speaker still seems sure of its own value, even if no one else has noticed it yet.