By Interstone Marble and Granite Limited
On the “Mohs” hardness scale where 1 is the softest and 10 the hardest, the following stones are reportedly as such:
Slate: 2.5 - 4
Marble: 2.5 - 5
Travertine: 3 - 4
Limestone: 3 - 4
Granite: 7 - 8
Quartzite: 7+
Based on the above scale, preferences are to use granite and quartzite on kitchen counter tops, and marbles and travertines in the bathroom.
Reference: http://www.steamboatcarpetsplus.com/c-1177-natural-stone-tips.aspx