Question:
I am trying to understand how hard quartz is. Is there a conversion to Rockwell hardness number?

Answer:
The Mohs’ Scale (Table 1) is used in mineralogy and gemology to determine the relative hardness of a material. Mohs is based the scale on ten minerals that are all readily available. Diamonds, as the hardest-known naturally occurring substance (when the scale was designed) are at top of the scale. The hardness of a material is measured against the scale by finding the hardest material that the given material can scratch, and/or the softest material that can scratch the given material. For example, if some material is scratched by apatite but not by fluorite, its hardness on the Mohs scale would fall between 4 and 5.

The Mohs scale is a purely ordinal scale. For example, corundum (9) is twice as hard as topaz (8), but diamond (10) is almost four times as hard as corundum. The table below shows comparison with absolute hardness measured using a sclerometer.

Thus, quartz (the second most abundant mineral in the Earth’s crust) is a 7 on the Mohs scale or approximately 178 Brinell, that is, about 90 HRB.

Mohs hardness values of other substances are shown here[1]:

0.2-0.3 – cesium, rubidium
0.5-0.6 – lithium, potassium, sodium
1 –talc
1.5 – gallium, strontium, indium, tin, barium, thallium, lead, graphite
2 – boron nitride (hexagonal), calcium, selenium, cadmium, sulfur, tellurium, bismuth
2.5-3 – magnesium, gold, silver, aluminum, zinc, lanthanum, cerium, lignite
3 – calcite, copper, arsenic, antimony, thorium, dentin
4 – fluorite, iron, nickel
4-4.5 – platinum, steel
5 – apatite, cobalt, zirconium, palladium, tooth enamel, volcanic glass
5.5 – beryllium, molybdenum, hafnium
6 – orthoclase, titanium, manganese, germanium, niobium, rhodium, uranium
6-7 – glass, fused quartz, iron pyrite, silicon, ruthenium, iridium, tantalum
7 – quartz, vanadium, osmium, rhenium
7.5-8 – hardened steel, tungsten, emerald, spinel
8 – topaz, cubic zirconium
8.5 – chrysoberyl, chromium
9-9.5 – corundum, silicon carbide (carborundum), tungsten carbide, titanium carbide, stishovite
9.5–10 – rhenium diboride, tantalum carbide, titanium diboride, boron
10 – diamond
>10 – nanocrystalline diamond (hyperdiamond, ultrahard fullerite)