<p>DURABILITY
The ability of gems to resist normal wear while set in jewelry is a perfectly understandable requirement  and  needs no  supporting  arguments. Durability depends upon mineralogical hardness, or the ability to resist abrasion, and upon toughness, or resistance to fracture. Some cleavable minerals, such as topaz, while quite hard and easily capable of resisting normal abrasion, are sometimes split in two by relatively weak blows. Others, such as jadeite, whose single crystals are easily cleaved, occur in compact masses where this potential weakness is overcome by the interlocking of numerous minute crystals into a material of astonish¬ing toughness.
The generally accepted rule of thumb for classifying gemstones as durable is that they be as hard or harder than quartz. The table below shows the Mohs Scale of Hardness which is used for gemstones as well as for ordinary minerals. It merely indicates which gemstones are harder than others and does not give absolute hardnesses.

Mohs Scale of Hardness
1.    Talc (softest)    6. Feldspar: moonstone,
2.    Gypsum: satin spar,    7. Quartz: amethyst, citrine, etc. alabaster    8. Topaz
3.    Calcite: cave onyx    9. Corundum: ruby, sapphire
4.    Fluorite: “blue John”    10. Diamond (hardest)
5.    Apatite

Many authorities state that all gems will eventually be abraded by atmospheric dust and the traces of soil unavoidably brought into homes and buildings because these contain fine particles of quartz (hardness 7), and unless gems are harder than quartz, damage will result. However, the severity of this kind of abrasion has never been convincingly demon¬strated, and it is the view of other authorities that it is very minor in effect, if it has any effect at all, and that most damage to gems results from careless treatment while being worn, or from allowing gems to bump into each other in jewelry boxes or drawers.
While durability has its unchallenged virtues, exceptions to the rule are made if the gemstone happens to possess some other outstanding quality. Perhaps the best example is opal, which is both a soft and brittle mineral (hardness 5 to 6 1/4). It is so liable to damage while being worn that jewelers are frequently called upon to demount an opal from a customer’s ring and send it off for repolishing. Despite this handicap, opal commands very high prices in fine specimens because no other gemstone can match its distinctive beauty. Other soft and weak gemstones which are considered acceptable for wear because of their outstanding beauty are moonstone, peridot, and the synthetic gemstone strontium titanate.