What are precious and semi-precious gems? According to Galleries.com, gems are stones or stone-like items which are prized as ornaments, particularly for the fashioning of jewelry. Most gems are minerals such as diamonds, fluorite, and lapis lazuli. Some gems have biological origins, such as amber and pearls. Also, a few are stones but not technically minerals. The opal is one of these.
Gems are often divided into “precious” and “semi-precious” categories. The division is based on a combination of beauty, color, hardness and rarity. Precious gems include diamonds, emeralds, rubies, and sapphires, according to Max Bauer in his book “Precious Stones.” Some people include pearls in this category, even though they are not actually stones. Until relatively recently, amethyst was also considered precious because of its rarity. That changed when a large quantity of high-quality amethyst was discovered in South America (Brazil). Minerals.net defines a gem as semi-precious if it is not rare, hard, and beautiful. Amethyst is still as beautiful and hard now as it was in the time of Aristotle, but it is no longer seen as rare. In theory, this should mean that synthetic rubies used for jewelry purposes should be considered semi-precious: the stone itself is as lovely and hard as any natural ruby, with which it is or should be chemically identical, but it can no longer be considered rare.
Beauty and rarity might seem reasonably obvious factors for determining the value of a rock, for ornamental purposes. Rarity simply points to the basic economic idea of supply and demand. Without beauty, what would be the point in using the rock as personal adornment?
Why is hardness a concern? As Max Bauer points out in his book “Precious Stones”, jewelry ought to last a long time, and some kinds of jewelry get some hard wear. This is especially true of rings. Ideally, gems should be relatively unaffected by the oils of the skin and should not be easy to scratch or otherwise mar. Therefore, part of being “precious” involves not being easily scratched by sand. Most sand is quartz, and so precious stones tend to be as hard as or harder than quartz. The recently-demoted amethyst is a type of quartz. Diamonds, sapphires, rubies and emeralds are all harder than quartz. Once again, the pearl is the odd man out. According to the merchants at GemSelect.com, harness is not the only factor to consider when figuring out whether a stone will last. A precious stone should be stable in light, moderate heat, and humidity.
The short version is that a precious gem has all of the “precious” qualities: it is durable, it is beautiful, and it is rare. A semi-precious stone has one or two of these traits, but not all. As they say at the fair, “Close, but no cigar.”