To understand pearl values, let's start by talking about... diamonds.
Like many of us in the jewelry industry, I have dealt with diamonds throughout my whole career. After all, even a pearl-jeweler uses diamonds to enhance and accompany pearls! So I intimately understand the grading system, cost structure and value of different kinds of diamonds.
But I can't imagine a consumer who doesn't have to do so professionally really understanding that system. Why would you? It goes far beyond the basic "4c's" - themselves not always so clear. Diamond value is a bit arcane. Yes, part of it is the 4C's. Part of it is also how the diamonds are cut. "Single cuts" are different than "Standard cuts"... one-pointers are a market unto themselves. How the diamonds are bought - Melee, Sites, Bourses, Lots, etc. - affects their value. Which entity graded the diamond factors in... a GIA certification is different than one by EGL, IGI or the various other folks who grade diamonds. Parts of the system just flat out don't make sense, even to me. Plus you have a supply/demand component where "less valuable" diamonds can command premium prices just because of supply issues (e.g. when "pave" styles got hot a while back, teeny-tiny diamonds all of a sudden got disproportionately expensive).
Pearl Values, on the other hand, make clear sense to me.
Pearls range from quite affordable indeed to highly extravagant. But there's always a reason. And that reason is pretty much always rarity.
Let's start at the top. Conch pearls are quite simply extraordinarily rare. Very few conch produce pearls, very few of those are gem-grade, and there's just not many of them around. That's why they're uber-expensive.
Moving down the list, Golden South Sea Pearls are also quite expensive... and quite rare. They're produced by only one type of oyster and that in a fairly limited area. Again... they're valuable because there are very few of them at good quality.
Tahitian pearls and White South Sea Pearls are a bit less dear than Goldens... But again, it's a straightforward price/rarity issue. Both types only grow in certain types of oysters in certain types of water.
Next in both rarity and price are Akoya pearls. Although they too are produced by a particular mollusk only in certain areas, the fact is the oyster that produces them is more abundant and the range of waters in which they'll grow is broader. So you can farm more of them and they are less rare... thus less expensive.
Last there are freshwater pearls. Produced by Mussels in freshwater ponds. Freshwater pearls benefit from the fact that while oysters normally produce only one pearl at a time, mussels can produce several. Couple that with the fact that there are multiple mollusk species that produce freshwater pearls, and that their range of home waters is broader, and you have a product that is relatively more abundant... and affordable.
There is, of course, also a qualitative factor. Nothing else looks like a conch pearl. The color of golden south sea pearls is truly stunning. The rainbow spectrum of colors in a Tahitian pearl is amazing. Akoya pearls have a deep and silvery luster that's unmatched. Etc.
But all that aside, one thing remains true regardless: Pearls are beautiful. They are true, unique creations of nature and have a magical quality to delight. In the end, they're valuable because we love to adorn ourselves with things of such beauty - the gifts of the sea. It is that which makes any pearl valuable. If pearls were drab and ugly, no amount of rarity would make them valuable. I'm quite sure that there is somewhere a type of mud that is extremely rare, found in only one place under certain conditions, and extremely hard to obtain... But generally, nobody's stringing mud on a rope to wear no matter how rare.
Having said all that, I'll risk self-promotion to say this: There's one other factor - where you buy your pearls. Being assured of a good-quality pearl is key. Working with a reputable seller - to whom you can talk if you want, e.g. - is a big plus. So is working with a brand-name pearl creator: someone with a brand reputation to uphold. So yes, a Conch pearl remains more valuable than a Tahitian which is more valuable than an Akoya, etc. But in the end, Pearls.com was started to add that last factor: a seller AND a brand that stands behind the quality and value of what you buy.