Physical Properties
Opal measures 5.5 to 6 on the Mohs scale meaning they are not particularly hard, which is the reason why many opals are set in a protective setting. Opals contain water (between 2 and 6%) and can dry out and become brittle. Drying out can be easily combated by regular wear, as the humidity from the air and body will keep it hydrated.
Tourmaline measures 7 to 7.5 on the Mohs scale so is reasonably hard and has excellent wearing qualities. Tourmalines have basal cleavage, so with bigger and thinner stones care must be taken not to subject the stone to hard knocks and bangs.