Lost Gem - Unfullfilled Promise

In the summer of 2010 I want to host a treasure hunt and reward prize if successful in Northeastern Ohio. Has anyone done this? If so any advice or pointers? I have a maximum search area of 10 acres starting and radiating out from a central location and the treasure may not be found due to time and odds. The short story is...the year my dad proposed my mom said no. In his upset some time later he tossed the diamond ring into what was to be the yard of their future home. They did get married and were happily so for 17 years. From day one dad promised to replace my moms ring at 25 years. He died in a plane crash feet from our property and the ring was never found nor was his promise to give her a new diamond. If that ring can be found or was meant to be found...I want to find it and it may take an army to do it. He has been dead for 17 years and I want to finish his unfinished business if it can be done; returning her ring and recognizing their love. She has done so much for her community and lost so many precious people in her life. She deserves to have her diamond if it can be found. Larry's Diamond Project would be the name of the hunt. I want to offer a reward if it is found. Any ideas on this would be greatly appreciated. From set up to tips for success... Thank you all for your support!