Soapstone is naturally a softer stone, since it is made up mostly by Talc, among other minerals. What makes soapstone softer or harder, is basically how much Talc is in it.
Average architectural grade soapstone , used for countertops, wood burning stoves , sinks, tiles, etc. will have around 50% of Talc, the harder ones, perhaps only 30%. Soapstone that is used for carvings, and the art industry will usually have around 80% of Talc.
Some of these really hard stones out there, have no Talc in them and in petrological terms cannot be called a soapstone, they are serpentinites. A serpentinite is what soapstone used to be, a metamorphism of serpentinite became soapstone. Basically if a “soapstone” is so hard that it cannot be scratched by a knife, chances are it is not soapstone, it is a serpentinite. What does that mean? Well as an example, one cannot use serpentinite in an application to deal with extreme heat, because it does not have the thermal properties that soapstone does. In a kitchen environment, serpentinite will probably work just fine.
I have heard terms like “the new generation” of soapstone being harder, which I get a kick out of. In geological terms, these serpentinites are actually the “old generation”, since the harder serpentintite metamorphised into soapstone, which is softer.
About the Moh’s Hardness Scale
Although the Moh’s scale was created with and classified using 10 basic minerals, not rocks or stones (which are not minerals) it actually can be used as a guideline to determine hardness of all materials, including stones.
To determine where any material (not only stones or minerals) falls on the Moh’s scale of hardness, we would need to see if it gets scratched by a mineral listed on the Moh’s scale(or another material that hardness has been pre determined). As an example, a fingernail cannot scratch Calcite(3) but it will scratch Gypsum(2), so it is considered that a fingernail is 2.5 or so on the Moh’s scale of hardness.
With that being said, if we take something that we already know it’s hardness and try to scratch a soapstone, and it does, we will know that the soapstone is at least softer than what scratched it. So if a fingernail scratches it, it can be assumed that, that soapstone is a 2.5 or less on the Moh’s scale of hardness.
If knowing the Moh’s value of a giving material is important to you, you can actually figure this out by yourself by trying to scratch it with the following materials of already known hardness:
fingernail = 2.5
a copper penny = 3.5
knife blade = 5.5 (Anything harder than this, is definitly a serpentinite and not a soapstone).
About Scratching Soapstone
What happens if you get your soapstone scratched? While most scratches will occur if one purposely tries to scratch their stone, if you get a scratch, the beauty of soapstone is that you can easily sand it off, with a regular sandpaper or even just a dab of mineral oil or soapstone wax will hide it.
Thanks for reading,
Rogerio M. Teixeira