| There are vast differences in the value of various gemstones, with some, like diamond, worth many times their weight in gold, and others that derive their worth chiefly from the effort it took to process them. In this node, we'll take a look at the cause of this massive disparity.
We'll start by assuming that a desirable gemstone is likely to be more expensive. This is simple economics. And yes, I realize
that this is a crude assumption.
Firstly, most valuable gems are shiny, a property they share with precious metals. The most
common reason for this is that the gem is transparent, and has a high index of refraction. This
means, that if the gem is properly cut and polished, the light will refract from the
interior edges and come back to the viewer, probably with its colors shifted like a miniature
rainbow. Small changes in the angle of the stone will show the bundles from different facets,
which will look very different. This will treat you to a beautiful sparkling light show.
Now, let's see how this pet theory will hold up to scrutiny, by looking at the indices of
refraction of various precious stones:
- 1.8 Corundum(Includes ruby and sapphire)
- 2.4 Diamond
- 1.6 Beryl (Emerald)
- 1.6 Topaz
- 2.0 Zircon
The average mineral has an index of refraction between 1.4 and 2.0. This explains the popularity of
diamond and to a lesser extent sapphire. Zircon is a popular imitation diamond, and with its high
index of refraction, is quite pretty. Still, the most valuable gem, emerald, isn't too special in
this department.
It's clear the shininess isn't the whole story. Pretty colors have an important role as well. The
deep, reflective black of the tourmaline, the vibrant, impossibly intense green of the emerald,
the red of the ruby... the stuff of poetry. For stones like emeralds and rubies, a beautiful
color is very important for the value. Combining this with the "shiny" argument that was made
earlier, we have a pretty good idea why very precious gems like sapphires, emeralds, rubies, and
diamonds are so valuable.
The previous arguments boiled down basically to "It looks pretty". But that doesn't explain why a
sphene or a fluorite, which are very beautiful stones by the aforementioned criteria, are
relatively worthless. This is because these minerals lack the hardness
necessary to maintain a nice, polished finish in practice. Gems typically will encounter objects
with a Mohs hardness of up to about 6. This places a lower limit on the
hardness of a suitable gem. And indeed:
The more precious stones are clearly at the bottom.
Another important thing is the size and perfection of the gem. In a transparent gem, small errors
are clearly visible, and can greatly diminish the prettiness and hence the value of the stone.
And, as always, size matters. More of a good thing is obviously better, but because you can't
just paste two gems together to make a bigger gem, the value of a gem increases strongly when a gem
gets bigger. Bigger gems are more rare. Linked to this is the shape: a transparent gem needs a
certain shape to sparkle optimally.
This brings us to the other part of the economic law of supply and demand. A rare gem will fetch
a higher price than a common gem of similar prettiness. Think about the huge difference in value
between zircon and diamond. A secondary source of demand, such as industry, might also influence
the price. Price fixing by the De Beers company keeps the price of diamonds sky high.
Another thing that makes gems wanted is their high price. It's a good way to flaunt your wealth,
which appeals to some. This means that valuable gems will get in higher demand, hence increasing
the value. This may lead to interesting bubbles, which subsequently crash, for instance if a new
mine is opened. A good way to lose money.
Summarizing, there are many aspects that make gems desirable. In the end, it boils down to the fact
that humans like colorful, shiny, unblemished, valuable trinkets, the bigger the better. I'm not a jeweler, so there might be more to it, but this seems to explain most that's behind it.
Sources:
http://mineral.galleries.com/minerals/property/ | 
