The third aspect of durability, in addition to hardness and toughness, might be called STABILITY. It relates to the ability of a gemstone to resist loss of color dissolution by chemicals, or other forms of deterioration. For example, pearl is damaged by fairly weak acids, some turquoise tends to become greenish when it absorbs skin oils and acids, and a number of gems tend to fade in color over a period of time. However, most of the important gems are very stable, since they resist even concentrated acids and do not lose color.
The stones that may give trouble include heat-treated zircon which tends to revert to a brownish color, some amethyst and kunzite (both of which tend to fade), plus turquoise and pearl (for the reasons mentioned above). With the exception of those changes involving loss or change of color, avoiding deterioration requires only a need to take care of the stones not reasons for avoiding their use as gemstones.