RIDDLE: What am I?

See if you can solve today's poetic riddle. Let us know how you did in the comments below, and feel free to share with your online pals too. HINT: If you are treasure hunting, only the Pinctada are valuable.

Rough and gray as rock,
I'm plain as plain can be.
But hidden deep inside there's great beauty in me.
What am I?

Answer Below