In the early 1800's Fredrich Mohs developed a simple gemstone hardness scale to help mineralogists identify stone types. His scale ranges from 1 to 10, with Talc being a one on the scale, Gypsum a 2, and at the upper end, Sapphire and Ruby are 9, while diamond is a 10 hardness.
Basically way the scale works is when two stones are rubbed together, the higher hardness stone will scratch the lower hardness stone, and the higher number will remain undamaged.
For home use, a pencil lead is about a 1 hardness, a fingernail about a 2.5-, a copper penny is approximately a 3.5-, a knife blade about a 5.5, and an unglazed porcelain plate about a 7.
Since dust particles are mainly quartz, one should use caution when wiping dust off of softer stones such as opal or turquoise, as the dust particles will scratch the softer gemstone material when they are rubbed against one another.