In the past, stones that did not have a certain “gemstone hardness” were generally referred to as semi-precious stones. Today this term is rarely used commercially, as it can be misleading; generally one refers to jewels or jewellery stones.

To be a true gemstone, a jewel needs to have a Mohs hardness rating of at least 7.

1. Hardness and toughness  - Mohs hardness rating is used to measure the hardness of gems and minerals.
These figures are based on the relative ease with which a mineral can be scratched by other items. But Mors' hardness ratings can be misleading. The distinction between the hardness levels of the minerals imay not be consistent.
For example, diamond is only one grade higher than corundum, but its hardness is many times harder than corundum. Red corundum (rubies and sapphires) is relatively hard, with a Mohs hardness of 9. It has good toughness and no splitting. Splitting refers to the characteristic that a gemstone breaks along a certain direction after being stressed. This makes it a great choice for rings and other everyday wear jewelry.

2. Stability in the case of normal wear, corundum is very stable
This means that it is heat-resistant, light-resistant and unaffected by general chemicals. Borate powder will corrode untreated gemstone surfaces. Gems that have been crack-filled, hole-filled, and dyed can be damaged by acids, even mild ones like lemon juice.

3. Using Clean warm soap water is generally safe.
Ultrasonic and steam cleaning devices can generally be used to clean untreated, heat treated and lattice diffusion treated gemstones. Materials that have been crack-filled, hole-mark-filled or colour dyed shall be cleaned with a damped cloth.
    
4. Gems treatments
Gems treated with lattice diffusion will have varying degrees of color penetration. In some gems, the treated color permeates the entire gem, while in others the color permeates only the surface.
For lattice diffusion treated gemstones where the color penetrates only the surface of the gemstone, surface damage or secondary cutting will remove the color.
Gems treated with crack filling will have cracks that touch the surface, will be mainly filled with glass with high lead content. Fillers can be damaged by in contact with a variety of chemicals. Even relatively mild substances, such as lemon juice, can cause changes in glasses with high lead content.
Sapphires with this filling method are much smaller than rubies, but in some cases the amount of glass filler in sapphire is quite extensive. Sapphire treated in this way requires extra care than untreated, heat-treated or lattice diffusion treated sapphire.