A student scratches a rock sample with his fingernail and leaves marks on the rock. What can be determined about this rock?
1 answer
The rock is likely to be relatively soft, as compared to minerals like diamond or quartz which are much harder than human fingernails. The scratching of a rock with a fingernail resembles the Mohs hardness test, which is used to determine how hard a mineral is. In general, harder minerals (with higher Mohs scale numbers) cannot be scratched by softer minerals (with lower Mohs scale numbers). So, if a rock can be scratched by a fingernail, it probably has a hardness of 2 or less on the Mohs scale, indicating it is likely composed of softer minerals such as gypsum or calcite.