Many Connecticut gravestones are carved from native red sandstone. Unfortunately, this material is not particularly durable — it is vulnerable to spalling when moisture infiltrates the tiny cracks and then expands by freezing. In its most fragile state, sandstone can crumble at the lightest touch. Whole sections can slough off at a time. Graveyard visitors shouldn't really take rubbings of any stone, especially red sandstone.
Here is what happens to red sandstone over time:
Stage One: Minimal Damage
Stage Two: Largely Illegible
Stage Three: Disintegration
Stage Four: Collapse