Great idea but from what I know about these processes, the item or items must be totally air free before being encased in the substance. If they are not, they will expel air bubbles into the compound's mixture while it is still in liquid state and when hard, the air bubbles are there and will cloud, distort and otherwise distract from the beauty of the objects sealed inside. If and only if, the air can be removed from the coins and their' encrustation, should you have anything like this done! Also, as Nutriasub stated, some will turn yellow and others will crack. Synthetic Clear (White) Sapphire would be my substance of choice! However, I do not believe that there is a way to get them encased into a growing Sapphire Crystal but who knows. It is worth exploring, since Sapphire with a hardness of 9.0 on the Mohs scale, when grown and then cut in the right manner and angles, will almost be indestructible, will literally last forever and never yellow!
Also, thanks for the earlier thoughts of sympathy! Diabetes and several bouts of Pneumonia have taken their toll on her' bod. She has Congestive Heart Failure due to excess fluids around her' heart and in her' body, a portion of her' heart is actually not functioning and considered dead and she is also in Renal Failure! Besides the medications for removing fluids retained in your body, they are trying daily Dialysis in hopes of removing the fluid but so far, it has not been very effective. We are hoping for the best but the third Dialysis treatment today, will have to alleviate the excessive fluid problems, otherwise there is very little hope. She seemed to have given up after our' oldest sister passed in December of 2011 but after some very emotional talks with all of us in tears, she is actually fighting for her' life now.
Frank
P.S. Everyone, please do not let my sister's health problems from detracting from this amazing ongoing story and Thread!