Identification test #1: the scratch test

The scratch test, a centuries-old technique, is an invaluable tool in the field of gemmology and sculpture. It involves gently scratching the surface of a material with a harder substance to assess its relative hardness using the Mohs scale.

This simple but effective method can reveal vital information about the durability and authenticity of gems and sculptures. In this article, we’ll explore how to use the scratch test on gems and sculptures and why it remains a crucial practice for experts in these fields.

Understanding the scratch test

The scratch test is based on the principle that a material (your glasses) can be scratched by a substance harder than itself (whatever’s in your bag). Hardness, in this context, refers to a material’s resistance to being scratched or abraded.

The Mohs hardness scale, as mentioned in a previous article, provides a standard reference for mineral hardness, ranging from talc (softest) to diamond (hardest).

This test is used in large part to narrow down the list of potential materials on unpolished, rough materials and minerals or sculptures that are not easy to test otherwise.

It is dangerous to practice on polished surfaces or cut gems, as the surface is definitive and any scratches caused will be too.

Rules for carrying out the scratch test:

• Never test on an object of historical or significant value. The scratch test is destructive and can irreversibly damage valuable objects.
• If a test is to be carried out, always scrape the non-visible base or an inconspicuous area of the object. This minimizes any potential aesthetic or structural damage.
• Perform this test under the magnification of a magnifying glass or microscope, so that you can see even the slightest micro-scratch.
• This test is destructive, so do not use it on cut gems or on the visible surface of finished sculptures intended for display. Reserve it if possible for uncut gem specimens or raw materials used in sculpture.

Tools for scratch testing:

• A set of minerals with known Mohs hardness values (commonly known as “scratch test kits”). There are pen kits with tips graduated from 1 to 10, which make measuring much easier (and save you from carrying talcum powder around in your pocket).
• The gem, sculpture or mineral specimen you wish to test.
• A clean, flat surface on which to perform the test.

Test process:

Select the reference mineral:

• Start with the softest reference mineral, such as talc (Mohs hardness of 1).
• Gently scratch the surface of the gem or mineral specimen with the reference mineral or pen.
• Observe whether the reference mineral leaves a visible scratch mark on the surface of the specimen.

Repeat the process:

• If the specimen shows no visible scratch marks, move on to the next harder reference mineral (Mohs hardness of 2).
• Continue this process until you find the reference mineral that successfully scratches the specimen.
• The Mohs hardness of the specimen is approximately equal to the hardness of the reference mineral that successfully scratches it. If the specimen tested is scratched by a mineral with a hardness of 7, then we can conclude that the specimen has a hardness value of approximately 7 on the Mohs scale.

Record and interpret your results:

• Document the results, noting which reference mineral produced the scratch.
• This information can help identify the gem’s hardness and facilitate appropriate grading. In our example of a specimen with hardness 7, it will then be possible to draw up a list of potential materials, which will facilitate the testing process.
• Bear in mind that some gems or sculptures may be made of more than one material, so test in different areas if necessary.
• It should be noted that it is not advisable to come to a definitive conclusion about a material based on the scratch test alone. In fact, it is possible to find a composite material or simply one of the many minerals that share the same hardness. It is necessary to continue with an evaluation of visual clues using the naked eye, magnifying glass or microscope, as well as more advanced tests such as observation using a refractometer, UV lamp or spectroscope.

Conclusion:

Scratch testing is a valuable technique for gemologists and sculptors, enabling them to determine the relative hardness of materials and make informed decisions about their authenticity, durability and suitability for various applications.

When carried out with care and precision, scratch testing reveals crucial information about the materials tested, contributing to the proper classification and preservation of gems and sculptures for generations to come.

However, it is essential to approach this technique with caution, and to consult experts in the field when dealing with high-value or delicate specimens.

Don’t forget to follow the rules of the scratch test to ensure responsible and appropriate use.

Please don’t use these “just for fun” tests on granny’s antique jewelry or great-uncle’s Ming vase.