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The 4 C's of Gemstones
Many people are familiar with the popular 4 C’s (color, cut, clarity and carat). This phrase was created mainly as an aid to help the average consumer understand how the value of a gemstone, particularly diamonds, is calculated. Within reason, each of these four criteria can be useful in grading all gemstones. Each category would carry different weight depending on the type of gemstone you are studying with the biggest difference being between colored gemstones and colorless gems like the diamond. For colorless gems the most important criteria is cut. This is the style in which the gem has been fashioned to show off its brilliance. For a colored gemstone the criteria of color and clarity would hold more weight than the value of its cut.
Mineral Hardness and the Scratch Test
Another interesting method to determining gemstones and their relative value is their hardness as compared to other gems and minerals. Hardness is especially important in gemstones used as jewelry. The scale used to determine this factor is known as the Mohs Scale of Hardness. It was created in 1812 by the German mineralogist, Friedrich Mohs. Most gemstones have a hardness of 7 or higher on this scale. Very few soft minerals are cut and are usually done so for collectors but not for use in jewelry.
In order to determine the relative hardness of an unknown mineral you can use the scratch test. For example: if the mineral in question is able scratch a surface like apatite (a 5 on the scale) then you know its hardness rates above a 5.0. However if the mineral in question is able to be scratched by apatite then you know its relative harness is below a 5.0. Additional tests would have to be done to narrow down the exact hardness of the mineral.
Mohs Scale of Hardness is only relative. Just because diamonds are listed at a 10 and topaz is listed at 8, diamonds are actually 8 times harder than topaz! An absolute hardness scale would have larger differences between each of the levels.
Mineral Scales of Hardness
| Mineral |
Mohs |
Knoop |
| Talc |
1 |
1 |
| Gypsum |
2 |
32 |
| Calcite |
3 |
135 |
| Fluorite |
4 |
163 |
| Apatite |
5 |
430 |
| Feldspar |
6 |
560 |
| Quartz |
7 |
820 |
| Topaz |
8 |
1340 |
| Corundum |
9 |
1800 |
| Diamond |
10 |
7000 |
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Mohs vs Knoop Scale Diagram
Click Image to Enlarge
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Examples of Common Items and Their Relative Hardness on the Mohs Scale
| Hardness |
Substance or Mineral |
| 1 |
Graphite or pencil lead, talcum powder |
| 2.5 |
Average fingernail |
| 2.5-3 |
Pure gold, silver or aluminum |
| 3 |
Copper Penny , limestone |
| 4-4.5 |
Platinum |
| 4-5 |
Iron |
| 5.5 |
Knife Blade |
| 6 |
Titanium |
| 6-7 |
Glass |
| 7 |
Quartz (ex: amethyst and tigers eye) |
| 7-7.5 |
Garnet |
| 7-8 |
Hardened Steel |
| 7.5 |
Tungsten |
| 8 |
Topaz, Emeralds, Aquamarines |
| 8.5 |
Chromium |
| 9 |
Sapphires, Rubies |
| 10 |
Diamonds |
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