Emerald, ruby, opal, gems and jewels-these words have been in our vocabulary for thousands of years. However, defining what a gemstone is has proven to be a major challenge.

Not for everyday people with common sense, but for those lexicographers who have a need to precisely define each work in our language.

Most gems are "minerals that have been chosen for their beauty and durability, then cut and polished for use as human adornment." This definition covers the vast majority of the things we regard as gems. The problem is that for every defining feature, there is an exception.

Most gems are minerals but some, notably pearls and amber, are organics. That means they were created by living organisms. The coating of a pearl is mineral which is confusing some folks. By definition though, a mineral must be created in the earth. Hence, pearls fall into a different category.

Durability is usually a high priority in choosing a gem, but two of histories most popular gems are particularly delicate. Pearls are generally considered to have a life span of about a century. That is because they are soft and the simple act of wiping the dust off of them slowly wears the surface coating away. And woe is be to the woman who puts her pearls on before using her hair spray and atomized perfume. These can seriously stain and damage pearls.