Do a goggle search on cleaning Lime Stone , but some people place coins under the stone to raise it if not to tilted. I had a similar problem with my Uncle's stone. It was (surface mount )laying flat . The stone was sunk down below level of the soil. I could not find or contact the Care taker of this smaller Cemetary in Litchfield Illionois . so my brother and I raised the stone up and added crushed rock from a pile of it at a corner of the Cemetary and set it back down at a proper height! I was his name sake nephew and I couldn't let it be ! I hope I didn't break any laws by doing so BUT If so I'll just have to live with it. My Uncle was a LT. Commander (Navel Air) during WW2 He also was a volunteer at Bikini Atoll during Operation "Cross Roads " when dropping the A bomb . I might add that he was also one of the many brave men and women that Volunteered to stand up and face the blast ! My uncle Russell E. Pearce died of lung Cancer in 1987 while I was in RVN . GOD BLESS THESE MEN / WOMEN and their family's for their sacrifice for all of us ! Certain coins have a special message / meaning to all that will see them if placed on top of a Tomb Stone !