The hardness of a gemstone refers to the scratch hardness and then to the cutting resistance. It comes from the strength of the chemical bonds. The Mohs’ Hardness Scale has 10 levels from 1 (softest) to 10 (hardest). The polish and luster of gemstones below 7 can be damaged, so care must be taken when wearing and storing. Gemstone hardness is also listed in half degrees. The levels are not evenly spaced. The diamond is only one number away, but it is many times harder than the gems in the corundum family.

1 Talc
2 Gypsum
3 Calcite (2.5-4.5 Pearl; 3.5-4 Malachite)
4 Fluorite
5 Apatite (5-6 Lapis, Turquoise; 5.5-6.5 Opal)
6 Orthoclase Feldspar (6-6.5 Moonstone; 6.5-7 Garnet, Zircon, Peridot, Tanzanite)
7 Amethyst, Citrine, Quartz (7-7.5 Tourmaline, Iolite; 7.5-8; Aquamarine, Emerald, Beryl, Morganite)
8 Topaz, Spinel (8.5 Alexandrite)
9 Corundum, Ruby, Sapphire
10 Diamond

(1 & 2 Can be scratched with a fingernail; 3 Can be scratched with a coin; 4 Can be scratched easily with a knife; 5 Can be scratched with a knife; 6 Can be scratched with steel file; & 7 Scratches window glass.)

Anything below a 9 is not recommended for ultrasonic or steam cleaning. Ultrasonic vibrations can weaken already fractured stones and hot steam can cause oil or unhardened resin to seep out of fractures. Using a recommended jewelry cleaner or warm, soapy water with gentle scrubbing is the safest way to clean gems below 9.