The Mohs scale of hardness is used to identify the relative hardness of a substance, from copper to glass to alabaster.  The scale based on the ability of a harder substance to scratch a softer.  Diamond is able to scratch steel, steel is able to scratch wood, wood is able to scratch chalk.  In the Mohs scale materials are assigned a level of hardness 1 through 10, with one being the softest material (talc) and 10 being the hardest (diamond).  If a material has a Mohs hardness of 5, that would means it would scratch a material with a hardness of 3.  If an as advertised as diamond (supposed to have a hardness 10) is scratched by steel (hardness 5), then it clearly is a fake.  And you’ve probably seen this diamond test done before in movies.
Along with other tests and observations (color, shine, weight, etc), the Mohs test is often used to identify a material and weed out imposters.

You can buy inexpensive Mohs testing kits on eBay and at amazon.  The kits simply contain 9 different minerals with a mohs hardness of 1 through 9.

You can also use many around the house items for quick reference, including glass, nails and pennies.
The following is a 1 through 10 list of different substances..

1 (softest): tacum, chalk
1.5: tin, lead, graphite
2: gypsum, plaster of paris
2.5-3: human fingernail, magnesium, gold, silver, aluminium, zinc, Jet (lignite)
3: calcite, US penny, copper, arsenic, antimony, thorium, dentin
4: fluroite, iron, nickel, iron nail
5: apatite, tooth enamel, volcanic glass
5.5-6.5: window glass
6:  titanium
7: quartz, steel file, ceramic tile
7.5-9: emerald, hardened steel, tungsten, garnet
8: topaz, cubic zirconiam
9: corundom, ruby
10 (hardest): Diamond

Purchase a copy of Identifying Common Materials in Antiques: A Pocket Guide