The ability of one material to scratch another is a fundamental physical property used to identify minerals. This characteristic, known as hardness, measures a material’s resistance to permanent deformation or abrasion. When geologists encounter an unknown mineral, they often test this resistance by attempting to scratch it with materials of known hardness. A simple scratch test can quickly narrow down the possibilities for identification by determining where the unknown substance falls on a standardized scale of relative hardness.
The Mohs Scale of Mineral Hardness
The most common system for quantifying this resistance to scratching is the Mohs scale of mineral hardness, developed in 1812 by German mineralogist Friedrich Mohs. This scale is an ordinal, or relative, measure that ranks ten common minerals from the softest (1) to the hardest (10). Each mineral on the scale can scratch all the minerals with a lower number, but it cannot scratch any mineral with a higher number.
The scale’s ten reference minerals are Talc (1), Gypsum (2), Calcite (3), Fluorite (4), Apatite (5), Orthoclase Feldspar (6), Quartz (7), Topaz (8), Corundum (9), and Diamond (10). The scale is not linear; the difference in absolute hardness between Corundum (9) and Diamond (10) is significantly greater than between any other two consecutive minerals. The system’s utility lies in its simplicity for comparative field testing, rather than providing an absolute measure of material strength.
Glass Hardness as a Testing Standard
Glass serves as a practical, readily available reference point for hardness testing in the field. Common window or bottle glass has a hardness between 5.5 and 6.0 on the Mohs scale. This makes glass harder than the reference mineral Apatite (5) but softer than the mineral Orthoclase Feldspar (6).
The glass surface acts as a threshold, separating minerals into those that can scratch it and those that cannot. Any mineral with a Mohs hardness of 6.0 or greater will abrade common glass. Conversely, minerals with a hardness of 5.5 or less will leave no permanent mark.
Minerals That Exceed the Glass Threshold
Minerals hard enough to scratch glass have a Mohs rating of 6 or higher. Common Feldspar, often rated from 6 to 6.5, is the borderline mineral that may or may not scratch glass, depending on the specific type of glass and the mineral’s exact hardness. Minerals with a definitive hardness of 7 and above will easily cut into the glass surface.
- Quartz (Mohs 7) is the most common mineral that easily scratches glass and is abundant in many types of rock and sand.
- Topaz (Mohs 8) is considerably harder and is often used in jewelry due to its superior scratch resistance.
- Corundum (Mohs 9), which includes Ruby and Sapphire, is hard enough to be used in industrial abrasives.
- Diamond (Mohs 10), the hardest known natural mineral, will scratch any form of glass, leaving a deep, permanent groove.
Why the Scratch Test Matters
The scratch test is a fundamental tool for field geologists because it requires minimal equipment and provides immediate results. The simple methodology involves pressing a sharp edge of the unknown mineral against the smooth glass surface and dragging it firmly. The crucial distinction is between a true scratch and a chalky, removable line.
A mineral softer than glass may leave a powdered residue that resembles a mark, but this line will wipe away with a finger. A mineral harder than glass, however, will produce a permanent groove or indentation that cannot be removed. This test quickly eliminates softer minerals, narrowing the search and allowing for more accurate identification.