Final answer:
Calcite is the mineral that could be scratched by a penny but not by a fingernail, according to the Mohs hardness scale.
Step-by-step explanation:
According to the Mohs hardness scale, a mineral that could be scratched by a penny but not by a fingernail is calcite. Calcite has a hardness of 3 on the Mohs scale, which is softer than a copper penny with a hardness of about 3.5, but harder than a human fingernail with a hardness of about 2.5.