Stones that are set into metal for the purposes of creating jewelry or art may be precious or semi-precious stones. The terms "precious" and "semi-precious" are defined in various ways by different organizations, but, generally, precious stones are of intrinsically greater value than semi-precious stones. Because there is so much variation in the interpretation of value, it is useful to understand the common approaches used when classifying types of stones and the problems that can be caused by putting too much weight on such a classification.
Regulations
The Federal Trade Commission (FTC) publishes guidelines for the jewelry industry on the use of terms such as "precious" and "semi-precious" in describing or categorizing stones. The guidelines are intended to help protect consumers from deceptive claims by limiting the use of such terms to particular, well-defined cases. However, the fluctuating nature of market demand, with some stones periodically becoming more popular and others less so, prevents the creation of a permanent and unchanging list of stones in a given classification.
Guidelines
Historically, stones that have been of particular cultural or religious importance have been seen as being of greater value. Similarly, the comparative rarity of a given stone and the market demand for the stone have influenced stones' classifications.
For example, amethysts were once comparatively rare and so were classified as precious. However, the discovery of large deposits of amethysts in Brazilian mining operations made the stone so commonly available that its status was changed to semi-precious.
Quality
Although a class of stones may be generically classified one way, the quality of a particular example of a stone can greatly influence its value. For example, diamonds are widely considered precious and aquamarines semi-precious. However, a diamond of poor quality and inferior cut could be of lesser value than a similarly sized aquamarine of exceptionally high quality and superior cut and polish. Therefore, the classification of a type of stone as precious or semi-precious, though useful as a general guide, can be of lesser importance than the quality of a particular stone.
Precious
The stones considered precious have changed over time and can even vary from country to country. However, a core set of stones are generally viewed as precious in most cases. These stones include diamonds, rubies, sapphires and emeralds. These stones have historically been rare and have been the object of study by craftsmen. As a result, the quality of cuts and the techniques associated with cutting the stones to show them to their best advantage has evolved over time and continues to evolve.
Semi-Precious
The list of semi-precious stones has likewise changed over time, and the categorization of a type of stone as semi-precious can be influenced by marketing techniques. For example, stones included in lists of "birth stones," where a stone is associated with the month in which a person was born, can be elevated temporarily or permanently to semi-precious status fairly arbitrarily. Stones such as jade, opal and turquoise may have enduring presence on semi-precious stone lists, while others, such as agate or coral, may be included on such lists temporarily or intermittently.
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