How hard is alumina on the Mohs hardness scale?

Alumina, which is synthetically produced aluminum oxide, has a hardness of 9 on the Mohs hardness scale. This rating places it among the hardest materials, as the scale ranges from 1 to 10.1

It is important to note that while the Mohs scale is useful for field identification of minerals, it is not a linear scale and is not always suitable for precisely gauging the hardness of industrial materials like ceramics. For more accurate measurements of such materials, scientists often use the Vickers hardness or Knoop hardness tests.2