Before you touch that tombstone.......... 
check out these links brought to our attention by Jan Ramos.

This is the web site for the Connecticut Gravestone Network,  The emphasis is on proper care, preservation, & restoration techniques that do no further harm.  It seems that a lot of well- meaning folks are hurting the old stones around the country - this includes some professionals hired by local cemetery associations for cleaning, etc.

The 2nd site is the Association for Gravestone Studies with emphasis on care, art, importance, symbolism, & more.

The word needs to get out - plain water, soft brushes only for old stones. No chemicals, no power washing (which is what the pros love to use - not only physical damage, but can change the color of the stone!).

I can't believe how much I've learned in a very short time thru contact with the former organization.  Glad I did a search & found them before I tried anything on the stones I was interested in.   
Thanks, Jan Ramos