Stone can be classified into different levels of hardness and in this way we get an indication of its workability.
For this work we’re really only concerned with Granite, marble, limestone and travertine the stone you’ll mostly come across if you’re making a copy of a Roman mosaic.
I. Granite is the hardest, to a point where it almost becomes too hard to work. It can be cut with a hammer and hardie but it requires more skill and effort and you need to factor in the extra time to resharpen your tools.
II. Marble comes next and obviously it’s the first choice due to the colour range and it is a good stone to work. It’s workability can though be affected by the veins in the stone which can direct the line of cut, ie the stone will start to cut where you want it but then finish along the line of the vein.
III. Limestone, a little softer so does not cut as well as marble. Some such as Crema Marfil, one of the shades I use for a background white, are marketed as marble but are in fact Limestone.
IV. Travertine, this stone has some nice, vibrant shades but has small holes in it which can cause it to crumble when cut. These holes will also take up the grout leaving small grey areas on the surface.
You will hear of the Moh’s scale (created by a German geologist Freidrich Mohs) used in reference to what crystal can scratch or be scrathed by another. It is though sometimes used as a rough guide to hardness of a stone. This is a scale of 1 – 10, 10 being the hardest of all materials, diamond, 1 being the softest, talc.
Mostly all you need to know is that marble is about 3 – 5 on the scale with granite higher. Granite being so hard will then polish up well which is why you see so many kitchen worktops made from it.
A good comparison is that your fingernail rates at 2.5, a copper coin 3 and a penknife blade about 5.5.