purveyor of pearls UK/EU
Aug 18, 2007
It has been bugging me for some time...why does nacre come in so many colours?

In a Darwinian sense it makes no sense to have such amazing colours on the inside of the shell, surely. No-one is going to see them, they won't attract a mate, they don't make that particular mollusc more successful and yet there they are, in the dark.

While there has been some tweaking and human intervention in the last 50 years or so, before that nature did all its own thing.

So...why? What is the advantage in having coloured nacre inside your shell?
Ah. That was a transcendant moment Mikeyy. It made for a beautiful wake-up with coffee moment. Seriously nice way to start a day.
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Great question, debated at P-G in the past under such banners as 'orient vs. iridescence' but here reduced to its bare bones. A quick observation is that the percentage among all mollusks that produce this phenomenon to the extent of viable pearl production is very low, making it a phenomenon indeed.

I just found THIS brief study of the nacreous biomineral process that should be useful for everyone here. The SEM image of nacre is of the gastropodal/columnar variety (not the terraced bivalvian brick-and-mortar), otherwise it is one of the best overviews I've seen.

But explaining color purely as the result of interference and diffraction caused by the nano-assembly and physical dimensions of nacre platelets leaves out the organic element, ie whatever genetically-induced pigmentation might be contributed by the conchiolin. Need to search a bit more for this?

Then you have the wildly colorful non-nacreous mollusks, primarily gastropods, that come in a huge variety of colors (think Melo-Melo and Queen Conch). How does that work without the benefit of interference/diffraction?
Just to add that in the grafting process the selection of mantle tissue (its anatomical location in the donor mollusk) is a factor in color. This would point more towards the organic/genetic component. Douglas, Mickeyy, Jeremy, Dave, (Jacques?) and other industry practitioners here might care to divulge a secret or two.
Steve, I'm not talking pearls - the inner shells of these molluscs are colourful and iridescent with or without pearls.
Steve, I'm not talking pearls - the inner shells of these molluscs are colourful and iridescent with or without pearls.
Wendy, pearls ARE the insides of shells, inverted.
Well, yes, obviously. I do know that. However there are more shells than there are pearls
I'm not asking how the nacre is made. But Why?
Every mollusc has a shell. some of those species's shells are lined on the inside with stunningly beautiful iridescent colours. Why?
The question goes directly to early molluscan phylogeny and the very purpose of the calcification process that began just 550 million years ago (out of 4+ billion since Earth came into being). Beginning as a means to keep hard minerals from fouling the body's soft parts, it quickly gained the primary purpose in molluscs of defense?the first arms race. As predators became more dangerous, shell materials were strengthened. From the first shells to nacre took about 40 million years according to the most recent studies.

Nacre production consumes a disproportionate amount of energy. Wherever possible molluscs have produced sheer bulk of non-nacreous material (think Giant Clam) as the preferred and easier task. Thus the minority of nacreous species, which the fossil record indicates to be continuing on the decrease.

Caitlin: You're absolutely right!!
What a great question!

Nacre is about strength and comfort to the animal.

Everything in the ocean is born swimming (and nothing dies of old age). In bivalves, once veligers (larvae) develop a foot, they produce byssus which helps them hold fast to objects, where metamorphosis can occur. This is the point in time where they need to build a shell. Being submerged in water, they need to produce a barrier so they can mineralize a shell by metabolic processes. This is the role of the periostracum.

Let's imagine for a moment, we have no home and it's pouring rain. The very first thing we'd do, is find shelter. An umbrella, a tarp, some branches etc. would do, so that we can go to work on building a home. The prismatic layers of a shell can be compared to bricks and mortar. Once the walls are up and the roof is on, we are going to need a place to sleep.

Surely we are not going to sleep on the floor. We'd get a mattress then put a sheet on it. This is the role of nacre. Some prefer flannel. some prefer satin.

Although most molluscs have similar process, they occur in varying degrees across species. Not all (in fact very few) are highly nacreous.

Regional distribution very often dictates the building materials used. If we lived in Canada, we'd use a lot of wood (floors, walls, even roofs), while in Mexico perhaps some concrete and palm fronds are utilized to the same end.

Oceanic mussels have thick periostracum, thin prismatic layers and medium nacre. Venus clams have thin periostracum, medium prismatic layers and thin nacre. Likewise freshwater molluscs tend to have thin periostracum, thick prismatic layers and thin nacre and so on.

Interestingly enough, subtidal and intertertidal animals of the same species behave differently, despite being at the same temperature and having the same food source. Beach oysters have thick shells and are easy to shuck without breaking, while suspended oysters of the same age group are thinner and crumble when shucked. This is adaptation at it's finest. The beach life requires protection against dynamic forces such as wave action, fresh water and predators. Suspended oysters need protection against parasites and other predators in markedy different degrees.

So what does this have to do with color and irridescence?

Let's examine abalone, one the most nacreous animals of all. The structures of prismatic growth are not entirely designed for strength insomuch as they are insulation. Abalone need strength to a greater degree, because they are not sedentary and as such are more vulnerable to larger predators. Aragonite is slightly harder than calcium carbonate, but with hardness comes brittleness. They use layers of protein to cushion these layers in an arch, hence increasing the breaking forces required to damage them.

Aragonite is translucent for the mostpart, but can appear nearly transparent at times, thus allowing light to pass and refract.
Aragonite is translucent for the mostpart, but can appear nearly transparent at times, thus allowing light to pass and refract.
Nacre offers the dual phenomenon of refraction and diffraction (nacre tablets being approximately the wavelength of light). Prismatic and crossed-lamellar shell structures are also aragonite, more primitive in origin than nacre and without the iridescence. Yet, they also offer a tremendous range of color.

Thus the need to address the organic element.
I have heard they are making artificial nacre -abalone type, I think. Does artificial nacre have the same prismatic/diffraction qualities as natural nacre?

"They" were talking about using it for car panels and hoods, etc. I immediately imagined abalone colored cars and I want one!!!!

Now I am really tripping! My abalone shell carapace and car, my mobile armor........why I could be a land-bound abalone equivalent in my abalone shell car. They could even style it after the shell shape. I wish i could draw one with the keyboard!
Artificial nacre: Maybe someone can come up with an image. I would doubt that the early attempts to mimic nacre's microstructure have resulted in anything remotely aesthetic.

My take from all the above is that color is a serendipitous byproduct of the microstructure and macro composition of nacre when exposed to light, totally without evolutionary significance. That it is produced in the absence of light attests to this.

So the question about the purpose of mollusc shell color would divert to gastropods and their non-nacreous aragonite. My assumption there is that the colors and patterns are intended as camouflage.
Prismatic and crossed-lamellar shell structures are also aragonite, more primitive in origin than nacre and without the iridescence... Thus the need to address the organic element.

Right Steve. Generally because of calcite. There are more than three hundred crystal forms identified in calcite and these forms can combine to produce a thousand different crystal variations. Calcite also produces many twin varieties. There are also phantoms, included crystals, color varieties, pseudomorphs and unique associations. There simply is no end to the varieties of calcite.

Aragonite is orthorhombic, while calcite is trigonal. Double refraction occurs when a ray of light enters the pearl and due to calcite's unique optical properties, the ray is split into fast and slow beams. As these beams exit the nacre, they bend into two different angles because the angle is affected by the speed of the beams. A person viewing nacre will see two images.

This is what we often refer to as "orient".
What possible evolutionary purpose does colourful nacre serve

As I mentioned in abalone, they evolved into a single shelled animal for added strength. The color itself is coincidental. Most molluscs don't have eyes or brains to process this kind of information.

On the other hand, cephalopods display and see colors, which indicate an evolutionary process.
Aragonite is orthorhombic, while calcite is trigonal.
The point being that non-nacreous aragonite still refracts, even while it does not diffract as in the layered nacreous configuration. But whereas refraction and diffraction are credited for the hues of nacre (leaving the organic component aside for the moment), would refraction alone account for the array of colors and patterns in gastropod (crossed-lamellar, non-nacreous aragonite) shells?