The term 'HIGH ON THE MOHS SCALE' refers to the Mohs Scale of Mineral Hardness, which is a scale used to measure the hardness of minerals. The answer to the crossword for this term is 'HARD', which has 4 letters. The Mohs Scale of Mineral Hardness is a relative scale that ranks minerals from 1 to 10, with 1 being the softest and 10 being the hardest. Minerals that are high on the Mohs Scale, such as diamond, are very HARD and difficult to scratch.