- 5 min read

Pink..it's my favorite color

I thought I was going to be a little late in jumping on the pink trend that dominated after the release of the Barbie movie but I still see it going strong so here’s my addition..a blog post about the science behind pink diamonds.

In the last post I discussed new research on how diamonds make it from far below the earth to the upper crust. It turns out that some of the rarest and most valuable diamonds, pink diamonds, may have made a similar trip but in their own unique way.

First, how do pink diamonds get their color? To answer this question, I ended up doing a deep dive into crystal formation. On our podcast, LuxeSci, we have talked about atoms and electrons and how atoms love to beg, borrow and steal electrons. In these cases, atoms love to share electrons and this sharing (attraction) forms bonds.

Crystals start to form when metals and non-metals are attracted to each other. This sharing of electrons creates an electron imbalance which attracts more molecules to the party. And this is where it all happens. How the atoms and molecules bond together influences the shape and characteristics of the crystal. Interestingly, where the attraction is located can determine the shape of the crystal. Dominant growth in one direction will get your prism and exclusive growth in one direction will yield needle-shaped crystals. High heat and where the crystal forms can also influence the shape. For example, the strength and direction of the flow in a solution can influence crystal shape.

Crystals don’t always grow all at once. If conditions change and one requirement for crystal growth is removed, the growth stops until the conditions are favorable again. This leads to different types of growth. Some crystals will grow as a whole and some grow in layers (twinning). This twinning phenomenon is most common in corundum and quartz.

Quick quiz: Do you know what gemstones are corundum?

OK, back to the science. Given all the heat and pressure around when most crystals form, it would make sense that the crystals themselves are under intense stress. This stress can introduce deformation of the crystal lattice and introduce strain (a measure of the deformation) in the rock. That stress and deformation of the crystal can impact its properties, including the color.

So what makes the diamonds pink? To help answer this, I found a cool paper that looks at two types of pink diamonds (E Gaillou et al, 2012). Diamonds from the Argyle deposit in Australia and from Santa Elena in Venezuela contain pinks bands alternating with colorless areas (the result of twinning) and are heavy strained. Pinks diamonds from other locations show more focused areas of strain that are localized near the pink lamellae (protrusions in the crystal that form a dendritic pattern).

The researchers hypothesize that twinning (growing in layers) results in plastic deformation (deformation of size and/or shape that is irreversible) that leads to new centers in the crystal and that these centers refract light differently and account for the pink color. What exactly are the centers responsible for the pink color is not yet known.

As we mentioned in the previous post, diamonds are formed deep underground and are likely brought to the surface through volcanic eruptions. A version of this process also happens for the rare pink diamonds. The Argyle deposit in Australia is where the vast majority of pink diamonds are from is a little different from most other diamond deposits.

While most diamonds are embedded in kimberlite, the diamonds of the Argyle deposit, which also include brown diamonds, are found in olivine lampriote. Lamproite is a sub-volcanic rock high in potassium and low in silica. Unlike kimberlite which is found only in a certain geologic time period, lamproite spans ages. It is formed from partially melted mantle below 150 km deep and is forced to the surface through volcanic pipes. The term olivine refers to macrocrysts within the lamproite. These are large crystal or mineral deposits ranging from 0.5 - 10mm.

Now the interesting part came when researchers from Curtin University in Perth, Australia went to measure the age of the stones at the Argyle deposit (HKH Olierook et al, 2023). Most diamond deposits in use today are located in archean cratons. I know that sounds like the name of an ancient Greek sea monster but what they are in geological terms are stable parts of a continent’s lithosphere (top 2 layers) dating back to the first 2 billion years of earth’s history. They are generally found in the interior of tectonic plates and have thick crusts with roots that go down hundreds of kilometers.

What is slightly different about the Argyle deposit is that it is found at an orogen (or orogen belt). These belts are created when a tectonic plate crumbles under another plate. This process lifts up the plate and creates mountain ranges. The process can also include magma coming up from layers below the earth’s crust. So this deposit of diamonds is at the edge of a plate as opposed to the interior of a plate, where most other diamond deposits are found.

Originally, the deposit was dated back to 1.2 billion years ago. The original researcher was not happy with that estimate because there was little tectonic activity in Australia at the time. So a team set out to re-analyze the dating data. They sliced extra thin slices of rocks and analyzed them to see what minerals were present to determine if they could use those minerals in the dating. The samples were then disaggregated into constituent minerals and analyzed using apatite U-Pb dating.

Essentially, this type of dating method uses apatite, which is a common constituent of igneous, metamorphic and some sedimentary rocks. To the best of my knowledge (so not the expert here), the decay path of uranium (U) and Thorium (Th) involves lead (Pb) daughter isotopes that can be measured and the amount is correlated to the age of the sample. So, measure the lead isotopes and get a good estimate of how hold a sample is.

Using this method of dating the samples from the Argyle deposit, the researchers found that the diamond deposit was from 100 billion years earlier than originally thought. This time frame corresponds with the break-up of Nuna (one of the earliest supercontinents). The break-up of the supercontinent would mean thinning along geological edges, which could have allowed for magma carrying the rare diamonds to come to the surface. This could represent an earlier phase of diamond surfacing. The hypothesis of supercontinent breakup triggering diamond eruptions also says that these eruptions ripple inland, like a pond when you drop a stone in. This would account for why most diamond deposits are interior and not at the edges of plates.

The last question is, why does this matter? Of course, for curious people like myself, it’s just really cool to have a better understanding of how these crystals are formed and how they come to the earth’s surface. I look down at the ring on my hand and am awed that this stone was formed billions of years ago way down in the earth’s mantle.

Curiousity aside, understanding the geology of how diamonds form and were brought to the surface can help target areas for future diamond mind exploration. Though, as always in science, there are still unknowns to figure, for example, where did all the carbon come from to form the diamonds? I’ll be sure to let you all know if I find a paper on that one.

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