Which rock type can be scratched with the fingernail?

The rock type that can typically be scratched with a fingernail is sedimentary rock, particularly soft varieties like limestone or chalk. These rocks generally have a hardness of 2 to 3 on the Mohs scale, making them softer than a fingernail, which has a hardness of about 2.5. This property allows for easy scratching and is a useful field test for identifying rock types.