Are gem-quality diamonds common?

Question

One often hears that diamonds aren't rare at all.

Upon closer inspection, such claims appear to actually state that diamonds cost more than their abundance alone would account for. Sources that outright claim that diamonds are common don't seem to have anything to back it up with. Plus, most sources that state numbers tend to state how many are mined, which is a meaningless number when one considers the possibility of artificially limiting the supply to inflate prices.

So, on a scale of iron being common (~50000 ppm in Earth's crust) and gold being rare (~0.005 ppm), are gem-quality diamonds rare in Earth's crust?

Answer 1

Upon closer inspection, such claims appear to actually state that diamonds cost more than their abundance alone would account for. Sources that outright claim that diamonds are common don't seem to have anything to back it up with.

In terms of where diamonds can be found, this is true.

Note that I believe a comparison of diamond production to gold production to be intrinsically flawed. The geological processes which create the deposits of each are vastly different. Gold is actually extraterrestrial in origin, whereas diamonds form from materials already on our planet over billions of years. There is a vast difference in the density of the materials, and gold's value is based solely upon purity of the metal, whereas determining value of a diamond is much more complex, and varies wildly from specimen to specimen.

A much more useful comparison is to other gemstones.

Mindat.org lists 738 localities for diamonds. Compare this to the 20 for benitoite, a very rare gemstone, the 381 localities for ruby, the 284 localities for emerald, or even the 626 localities for sapphire.

Note, however, that locality alone is a poor indicator, as the overall productivity of a mine (amount of desired resource retrieved) can vary significantly, and, given that the full extent of a site's production cannot be measured until it is fully exhausted (and even then it is possible to miss materials).

In 2013, rough diamond production from mines was estimated at 130 million carats. By contrast, in 2005, emerald production was estimated at 5,400kg (27 million carats).

Gem-quality diamonds only account for approximately 25% - 30% of the production of diamond mines*, which would mean that the global production of gem quality diamonds is estimated at between 32.5 million carats and 39 million carats in 2013. By definition, emeralds are gem-quality specimens of the beryl mineral family with a rich, distinctly green color, so 100% of emerald production can be considered gem-quality.

This puts global gem-quality diamond production at roughly 20% greater than emerald production.

Top-quality emeralds, sapphires, and rubies are rarer than top-quality diamonds, and can command a higher price-per-carat, but generally, other qualities of those gemstones are lower than comparable prices for diamonds.

It is difficult to evaluate comparative value vs comparative rarity when discussing gemstones, because the value of individual samples of gemstones varies tremendously based upon the specific characteristics of the specimen. In particular, color, size, and clarity (the amount of, or complete lack of, other mineral inclusions) impact the value of a given gemstone, although other factors such as cut, fluorescence, etc., can also increase or decrease the value.

*note that the proportion of "gem-quality" diamonds seems to be the topic of some debate. Between 3 sources (1, 2, 3), the proportion is respectively 20%, 25%, and 30%.

Answer 2

Total proved diamond reserves worldwide are on the order of 2.6 billion carats. This source provides numbers for some reserves in Russia along with their share in Russia's Reserves Register, giving us a figure of 1.29 billion carats for Russia alone. This same source also states that 80% of this amount is located in Yakutia, and that this amounts to "almost half" of the world's proved diamond reserves. Guessing ~40% as the "almost half" figure, we can infer that the total proved worldwide reserves are around 2.6 billion carats.

Note that these are natural industrial diamond reserves. Only 30% of diamonds are gem-quality. 2.6 billion carats is 520 metric tons, meaning there are ~160 tons of gem-quality diamonds in the known worldwide reserves.

By comparison, the USGS (among other sources) estimates (PDF) the unmined gold reserves at ~57,000 metric tons. Obviously these are only the known deposits, economically mineable – same as with diamonds.

Disregarding the estimated ~150,000 tons of gold already mined (and an unknown amount of gem-quality diamonds), unmined gem-quality diamonds are 300x-400x more rare than gold.

Both of these statistics look only at diamonds/gold that we can extract and hold in our hands. My interest in this started when lots of clickbait articles made it sound like in the absence of De Beers, diamonds would be literally worthless. Considering that they are shiny (when cut) and rather rare, they would certainly be worth a fair amount even without anyone artifically messing with the supply.

P.S. Another thing to consider is that diamonds, unlike gold, are not all equal. By color alone, only about 7% of the natural gem-quality diamonds are of the VVS clarity. D-color (colorless) internally flawless diamonds are estimated to total 0.001% of worldwide diamond production - on the order of 5-6kg in worldwide reserves. Obviously these are ridiculously expensive, De Beers or not.