Two strands that look similar in a photograph can differ tenfold in price. Here is where the difference sits.
Pearl pricing is a study in compounding rarity. Each quality factor is individually uncommon; a pearl that scores well on all of them simultaneously is very rare indeed, and a strand of forty such pearls that also match each other is rarer still. That compounding is why prices climb so steeply at the top.
The dominant factor within any type. High lustre requires fine, well-ordered nacre crystal, which depends on water temperature, oyster health and growth time — none of it controllable with certainty.
Determines both appearance and lifespan. Thicker nacre means longer growth, which means more risk and cost to the farmer, and a pearl that will still look right in fifty years.
Larger pearls need larger oysters, longer growth and carry higher mortality risk. Price rises steeply rather than proportionally: doubling the diameter can multiply the price several times over.
Perfect roundness is uncommon in nature. A truly round pearl commands a large premium over a near-round one that most people could not distinguish at a glance — which is precisely where value-conscious buyers should look.
Clean surfaces are rare because pearls grow in living animals under variable conditions. Minor marks are normal; a genuinely flawless surface is exceptional.
The hidden cost in any strand. To assemble forty pearls agreeing on size, shape, colour, overtone and lustre, a dealer sorts through many thousands. That labour is real and it is priced in.
The value buy. A near-round pearl with excellent lustre and a minor surface mark on the reverse costs substantially less than a flawless round one and looks all but identical when worn. Buy lustre, accept small imperfections, and you get far more pearl for the money.