According to the National Audubon Society Field Guide to Rocks and Minerals,granite, the most common material for modern tomb stones, has an average hardness of 6.5. Quartz, which makes up about 35% of granite, has a hardness of 7.0. Feldspar, which makes up about 45% of quartz, has a hardness of 6.0. The remainder of granite can be any of several minerals. Marble, another tombstone material, has a hardness of 3.0. Sandstone is made up of quartz granules (sand)

and a cementing agent calcite, or iron minerals. Calcite has a hardness of 3.0 and the iron minerals have a hardness of 4-6.

The hardness of steel is generally considered to be about 6.5, but can vary with composition and temper. A knife blade has a hardness of 5.5-6, while a file has a hardness of seven. A copper penny has a hardness of three. Any higher numbered hardness would scratch any lower numbered hardness. Slate is softer than a knife blade.

From this, it can be seen that quartz would scratch the steel, but that feldspar would be scratched by a saw blade or a file. Therefore, in my opinion, items such as knives, saws, wire brushes, or car keys should never be used to clean a tombstone. Rubbing a penny on a soft sandstone could damage the stone, or would leave an ugly streak on it.

For cleaning, I suggest water and the use of a bristle brush. This will do minimal if any damage to the much harder stone. True, if you work on stone long enough with a bristle brush, you might shine the stone a very small amount by the time you completely wear the brush out. The removal of the lichen is a great way to help preserve the stone -- they chemically and physically damage the stone.

( It has been brought to our attention that the use of shaving cream and other chemicals or concoctions on headstones is not recomended and can actually do more damage to the stone.  http://www.savinggraves.org/education/bookshelf/shavingcream.htm explains the damage that is caused by this method:

Many different things have been suggested for making the tombstone easier to read. One individual rubs the stone with fresh dirt. The sand in the dirt would scratch any of the stones. Another person uses chalk. Those of you in the more mature age groups can remember scratches on the slate blackboards from impurities in the chalk. Some have advocated the use of dream whip. This author can think of no substance in Dream Whip that would harm the stone. Many people have objected to the use of shaving crème, which is what I use.

I cannot find any possible way that shaving creme can damage a stone. You men have used it and know that it does not contain abrasives that will damage your skin; so, it will not wear the stone away. The big objection that I have heard is that shaving creme is acid and will eat the stone away. If you read the label, you will note that it does contain stearic acid.

The true acidity of any substance can be determined by measuring the H ion concentration in the substance. This is also known as pH. The pH scale ranges from 0 to 14, with 7.0 being neutral, less than 7.0 is acid, and more than 7.0 is basic. The cases that I am aware of, shaving creme ranged between 7.0 and 7.5 in pH. This says that there are no free H ions to react as an acid in shaving creme. Stearic acid is a long chain fatty acid. It is quite common in beef fat, but is in the triglyceride form. There are 18 carbons in the chain making up the acid. Stearic acid is nearly insoluble in water and is quite unreactive, except with heat, a catalyst, or with strong bases such as sodium hydroxide (lye).

The technique that I suggest using shaving crème is:

If you are on a genealogy trip, I would also suggest that you may want to carry a pair of pruning shears to remove brush that may be growing near a stone. If a stone has sunken into the ground, I cut the sod back from the stone with a pocket knife and remove the sod by hand to prevent damaging the stone with the knife. Some of you might also want a pair of gardening gloves.

It is hoped that the above information will help you make a wise choice in your method of cleaning and reading tombstones. Above all, please do not use any abrasive materials such as steel or rocks on the stone. The rest is your choice. Neither the author nor PHGS is advising you to use this method, just make a wise choice.