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.
