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.