Many people would say diamond that diamond is the most durable gem since it is the hardest of all gemstone materials. However, there is more to durability than just hardness. For example, diamond and topaz are both hard but exhibit perfect cleavage, which means they can be easily split by a single blow. Corundum gemstones, including ruby and sapphire, are not quite as hard as diamond, but they have no cleavage, a property it shares with spinel. This would mean that these gemstone types could be considered more overall durable when compared to diamond, even if they're not as hard as diamond. Other quite hard gems, such as emerald, may possess internal fractures that are filled by oil or resin; therefore, this makes them less durable since they require special care, despite being quite hard and resistant to scratching.
As a general rule, any gemstone with a hardness equal to or harder than quartz (7) are considered suitable for occasional wearing rings. If wearing the gemstone ring daily, it is recommended to choose protective-style settings, such as a bezel or channel set mounting. Gems with a hardness less than 7 can be set in rings, but should be worn with care and an occasional basis. These softer gems are better suited for gemstone jewelry designs such as pendants or earrings. Nonetheless, many softer gems, such as tanzanite and opal are quite often set into ring mountings.
It is also worth mentioning that there are a few softer gems with a hardness less than 7 that are extremely durable. Take nephrite and jadeite for example, the two main varieties of jade. Nephrite has a hardness of 6 to 6.5, while jadeite is slightly harder at 6.5 to 7 on moh's scale. Both jade gemstones are considered very tough due to their internal structures which have strong resistance to fracturing. Nephrite is especially strong due to its fibrous interlocking crystals with a matted texture. Jade often outlasts gems that are much harder, although an occasional repolishing may be needed in order to retain its beautiful luster over the years. For more information see our article on gemstone hardness and durability.