Someone gives you an unknown mineral. You can scratch it with quartz but not with feldspar. What is the mineral's approximate hardness?
To determine the approximate hardness of the unknown mineral, we can make use of the Mohs Scale of Hardness. The Mohs scale is a ranking system for minerals based on their ability to scratch one another.
Since the unknown mineral can be scratched by quartz (which has a hardness of 7 on the Mohs Scale), but not by feldspar (hardness around 6), we can conclude that the unknown mineral has a hardness that falls between those two values.
However, this method of comparison can only provide a rough estimate of hardness. To obtain a more precise measurement, specialized equipment (such as a hardness tester) would need to be used.