A gem Durability

Image result for What is a Gem?

Some minerals (such as those formed by evaporation of sea water) dissolve easily and clearly these would be poor gem materials.
Resistance to scratching: this is evaluated by consideration of gem hardness. There are two measures of hardness: scratch hardness and indentation hardness. Generally, we use the scratch hardness.
If we compare two different minerals, for example diamond and quartz (the main ingredient in beach sand) we will find that quartz crystals are readily scratched by diamond but diamonds can not be scratched by quartz. Thus, diamond is much much harder than quartz.
Commonly available materials can be arranged into a sequence of increasing hardness, e.g., talc-fingernail-copper coin-pocket knife-glass-steel file.
This can also be done with minerals. Moh arranged 10 minerals into a sequence that is known as Moh's hardness scale. This scale has talc (found in talcum powder) at the soft end and diamond at the hard end. The hardness of talc is 1, quartz is 7, diamond is 10.
Unfortunately, most minerals with hardness greater than 7 on Moh's hardness scale are brittle. Hardness is not toughness -- even a diamond can be broken.
Minerals can break by irregular fracture (like bottle glass) or by cleaving.
 Image result for What is a Gem?

The 4 factors that affect the value of a gemstone are easily remembered as the "4 c's":
·         Color: we will deal with the origin of color in gemstones in a separate lecture. Clearly, color affects value. Some colors are more desirable than others. In part, this is dictated by personal taste and in part by industry standards (e.g., for diamonds).
·         Clarity:  flaws (crack, inclusions) decrease the value of a gemstone.
·         Cut: the ideal proportions for gems (to optimize brilliance and fire) are not always to be found in a faceted stone. Poorly cut stones have much lower value.  Small errors in the placement of facets decrease the value of a gem. For example,
o        extra facets,
o        an off-center culet
o        or a gem with improperly pointed facets.
·         Carat weight: bigger is not always better, but for otherwise equal color, clarity, cut, the larger the stone will be more expensive!
1 carat = 0.2 g, thus 5 carat = 1 g <--- remember this!
 Notice that the number of carats depends on density, so two different types of gems of the same size will normally be a different number of carats!
Obviously, the rarity of a gemstone is an important factor in determining the value. However, some other things that affect value that are unrelated to the 4c's and rarity. The supply of a specific type of gem can be controlled to improve the value or a specific gem may greatly change in value due to consumer demand or perceived investment potential. It is interesting to look at the values of specific gemstones and see how these change over time.
The value of a gem may be much lower if its apparent clarity or color has been improved by treatment. Furthermore, synthetic gems (made by humans) have very much lower values than natural stones ... and beware! - the gemstone is not always the material it is claimed to be: it may be a simulant (look-alike). How do you know?
Many people turn to a professional organization such at The Gemological Institute of America (GIA) and American Gemological Laboratories (AGL) for the "final" determination, especially for more expensive stones. These organizations provide certificates that document the characteristics of individual gems.

No comments