Destructive Mineral Tests: Scratch Testing

Precautions

Scratch testing (or hardness testing) is one of the most destructive mineral tests. Never perform a scratch test on a finished gemstone. This can fracture or shatter the gem, even if tested on an inconspicuous area.

Gem rough or mineral specimens can be scratch tested, but caution is needed. Keep in mind that the rough may have incipient fractures that can't be seen or internal stress that will open up easily to pressure. For these reasons, scratch testing should only be performed on small protrusions. Whenever possible, saw a piece off of the main stone for testing.

What Tools Do You Need for Scratch Testing?

Scratch testing requires a set of pencils or rods with ends of known Mohs hardness value. You can buy hardness sets or make your own. Get some wood dowels about the size of a pencil. Attach small, sharp pieces of synthetic corundum, topaz, and quartz to the ends of individual dowels and label them 9, 8, and 7, respectively. For a more complete set, you can add feldspar and apatite, 6 and 5, respectively.

You don't need any other points because minerals with the same hardness won't scratch each other. Anything you can't scratch with a piece of corundum (9) must be a diamond (10) or a moissanite (9.25).

The majority of the specimens you'll work with have a hardness 7 or higher. You'll rarely work with stones softer than apatite (5), so you won't need testing points for the extremely soft minerals.

Test Procedures

To begin testing, select the scratching tool you think is the same hardness as your specimen. (Just take your best guess). With a moderately firm hand, try to scratch the mineral. Material rubbed off the hardness point resembles a scratch, so wipe the stone before inspecting.

You'll have one of two reactions:

• The stone is scratched: You now know that your point is harder than the mineral. Try the next softer tool and continue to work down in hardness until you find one that won't scratch your stone.
• The stone is not scratched: Try the next harder tool and continue to work up in hardness until you find one that will scratch the stone.

You'll finish with two numbers: the hardness value of the last tool that did scratch your stone and the hardness value of the last tool that didn't. For example, the 8 point might have scratched the mineral, but the 7 did not.

By definition, materials of the same hardness won't scratch each other. Therefore, we know that the mineral being tested has a hardness greater than or equal to 7 but less than 8.

What is the difference between gemology and mineralogy for scratch testing?

The difference between gemology and mineralogy when it comes to scratch testing lies in the objective. Gemologists are primarily concerned with determining the hardness of a gemstone for practical applications like setting a gemstone in jewelry. The durability of a gemstone really matters when it comes to different types of settings.

On the other hand, mineralogists use scratch testing as part of a more detailed scientific process to identify and classify minerals. They don't necessarily care about damaging their sample - the more destructive the test, the better they are able to make a class determination!

Moreover, while both gemologists and mineralogists use the Mohs scale for hardness, mineralogists may employ additional methods or tools (like mineral hardness kits, steel tools, or glass plates) to explore the relationship between hardness, composition, and crystal structure.

Editor's Note: The original article has been updated and adapted for the Mineralogy Certification course.

Donald Clark, CSM IMG

The late Donald Clark, CSM founded the International Gem Society in 1998. Donald started in the gem and jewelry industry in 1976. He received his formal gemology training from the Gemological Institute of America (GIA) and the American Society of Gemcutters (ASG). The letters “CSM” after his name stood for Certified Supreme Master Gemcutter, a designation of Wykoff’s ASG which has often been referred to as the doctorate of gem cutting. The American Society of Gemcutters only had 54 people reach this level. Along with dozens of articles for leading trade magazines, Donald authored the book “Modern Faceting, the Easy Way.”

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