If you had spent a small amount of time and energy propping up the general sentiment that your rock collection was the next big thing, yet had bountiful positive results in the the rock hype cycle, and the rock hype cycle regularly paid out considerable profits on such investments, I would indeed consider buying one of your rocks had I the $15m.

People regularly spend millions on sculptures which are indeed just special rocks. The Guennol Lioness comes to mind.

Then, if you made the rock cost a negligible amount of money given the regular payouts on such ventures, I would really start considering it. Don't be a boob.