What is Hardness?
Hardness is not only the condition of being a hard solid but in mineralogy it is a measurement of scratch resistance. In other words how resistant a solid’s surface to being scratched relative to another solid. A hard mineral can scratch the surface of a softer mineral. A soft mineral is unable to scratch the surface of a mineral that is harder than it. A diamond can easily scratch the surface of a fingernail but a fingernail cannot scratch the surface of a diamond no matter how hard it tries.
Hardness is generally a good thing. The harder the object the more valuable it is. The same is for turquoise, harder turquoise stones are generally more valuable than soft stones. Most turquoise found in nature comes in the form of chalk which is fairly soft. Treatment of the turquoise with enhancing agents such as oils and resins can be used to make the soft stones harder in a process called turquoise hardness stabilization. Good quality stones only need minor stabilizing if any at all. Naturally hard stones that do not undergo any stabilization tend to be worth the most.
Measuring Turquoise Hardness
The Mohs hardness scale is tool used for measuring the relative hardness of minerals, crystals and gems. The scale was invented by German mineralogist Friedrich Mohs in the early 1800s. (Binashaji) It was created as a method to easily identify an unknown mineral by quickly measuring its hardness. Gems and minerals are assigned a number between 1 and 10, 1 being the lowest and 10 the highest. This higher the number the harder the material. A fingernail is around 2.5 and a diamond is a 10.
Turquoise has a hardness of between 3 and 6 generally dependent on the quality of the stone and enhancement. As a point of reference a standard piece of glass has a hardness of around 5.5. Low quality natural turquoise called chalk can be scratched easily and can have a hardness of around 3. In order for turquoise to be properly used in jewelry it should have a hardness of at least 3. Stabilizing processes can be used the bring the hardness of low quality turquoise up to a 3.5 or more. Quality natural stones have a hardness between 5 and 6 without any help.
Scratching the stone
Determining the relative hardness of a sample turquoise stone involves either scratching the stone or using the stone to scratch another surface. This is called the scratch test. The scratch test does not take very long to do and is fairly accurate. The down side of this test can premaritally damage the turquoise sample. Unless you have extra stones from the same source or have the ability to buff out scratches from your sample I do not recommend this technique.
To do the test you will need a hardness testing kit. The kit includes 4 double sided scratching pens with points of graduated hardness from 2 to 9 and a few predetermined hardness plates. To test turquoise I used the scratching pens with hardness of 3, 4, 5 and 6. along with a plate of brass of hardness 3.5 and a plate of glass of hardness 5.5. I performed an example test using a low quality piece of turquoise that has undergone a poor stabilizing process and on old natural quality tumbled stone from a mine of the Royston District in Nevada.
First I stated with the low quality stone. I used the hardness level 3 scratching tip on the stone and it was unable to scratch the surface. Then I used the level 4 scratching tip and it easily scratch the surface of the stone. This lead me to believe that the stone has a hardness of more that 3 but less than 4.
Next I performed the same technique on the old natural stone. The scratching pens of levels 3,4, and 5 were unable to scratch the surface of the stone. The level 6 piece was able to make a fine scratch in the stone. This lead me to conclude that this stone has a hardness greater than 5 but less than 6.
I then used the samples to try to scratch the surfaces of the brass plate with a predetermined hardness of 3.5. The low quality stone was unable to scratch the surface only leaving a darkening mark on the metal (left). The quality natural stone however had no problem scratching the brass (right).
This concludes that the low quality poorly stabilized turquoise stone has a hardness between 3 and 3.5.
Moving on the the glass plate of hardness 5.5, I used the stones once again to try and scratch the surface. As expected, the low quality stone was unable to scratch its surface. However the quality natural stone was able to put a then scratch on the surface.
This leads to the quality natural stone having a hardness between 5.5 and 6 which is very high for natural untreated turquoise.
Value of a stone
Hardness of a turquoise stone is one factor of the turquoise quality index (TQI) that us used to determine the stones actual dollar value. Stones receive points a scale between 1-20. points are factored in to overall TQI.For stones that are obviously lower quality use the chart below(1-5). Brittle natural stones that are usually cloudy or stone that have been poorly stabilized receive (10) Stones that have been professionally stabilized are not allowed to receive more than (15) points even if they have a high hardness. Natural stones that seem dull and feel chalky are most likely under 5 on the hardness scale and receive (10). Rare high hardness natural stones may receive (20) points for hardness. If you cannot tell, then give it a (15).
|Hardness||plastic||chalk||reconstituted||Below 5 on MOHS scale||pro stabilization 5+ on MOHS scale||high natural hardness MOHS scale 5+|
|1 to 20||1||3||5||10||15||20|