Hardness Substance or mineral
0.2–0.3 caesium, rubidium
0.5–0.6 lithium, sodium, potassium
1 talc
1.5 gallium, strontium, indium, tin, barium, thallium, lead, graphite, ice[13]
2 hexagonal boron nitride,[14] calcium, selenium, cadmium, sulfur, tellurium, bismuth, gypsum
2–2.5 halite (rock salt)
2.5–3 gold, silver, aluminium, zinc, lanthanum, cerium, Jet (lignite)
3 calcite, copper, arsenic, antimony, thorium, dentin
3.5 platinum
4 fluorite, iron, nickel
4–4.5 steel
5 apatite (tooth enamel), zirconium, palladium, obsidian (volcanic glass)
5.5 beryllium, molybdenum, hafnium, glass, cobalt
6 orthoclase, titanium, manganese, germanium, niobium, rhodium, uranium
6–7 fused quartz, iron pyrite, silicon, ruthenium, iridium, tantalum, opal, peridot, tanzanite, jade
7 osmium, quartz, rhenium, vanadium
7.5–8 emerald, hardened steel, tungsten, spinel
8 topaz, cubic zirconia
8.5 chrysoberyl, chromium, silicon nitride, tantalum carbide
9 corundum, tungsten carbide, titanium nitride
9–9.5 silicon carbide (carborundum), titanium carbide
9.5–10 boron, boron nitride, rhenium diboride (a-axis),[15] stishovite, titanium diboride
10 diamond, carbonado
>11 nanocrystalline diamond (hyperdiamond, ultrahard fullerite), rhenium diboride (c-axis)[15]

Answer chromium