Gemstones are categorised for scratch resistance by referring to the Mohs scale of mineral hardness. The scale was introduced in 1812 by German mineralogist Friedrich Mohs. The scale goes from 1 to 10 with 10 being the hardest. A good choice for an engagement ring would be a diamond, a 10 on the scale or a Ruby or Sapphire, a 9 on the scale.
