Diamonds are the hardest substance, however they are not the toughest. Hence you cannot scratch a diamond with any other substance other than a diamond. But you can break a diamond with a hammer.

All are cubic Zirconia, as they are manmade chemical (Zirconium oxide). They are sold under these names.

Unfortunately it is a age old belief, as blue sapphire represents planet Saturn (SANI). Blue sapphires are one of the most beautiful and most popular gemstones found in nature, and sold world wide. This belief is common in India only, particularly in South.