Which stones are the best choice for engagement rings?

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.