The answer is fluorite. If a mineral cannot be scratched by a knife blade but can be scratched by quartz then its hardness is between 5 and 7 on mohs scale. A relative hardness value of 6.5 means that the mineral could scratch orthoclase but not quartz. Flourite hardness on mohs scale is 4, and therefore due to the very low hardness and perfect cleavage of Flourite, special care must be taken to ensure it doesn't get scratched or chipped. It cannot scratch the glass.
