Why pearl size is a spread, not a fixed fact of the type
A pearl farm does not produce one size — it produces a distribution, the same way an orchard produces apples across a range of sizes rather than one identical fruit. Each cultured pearl type has a full range that occasionally appears and a much narrower band that makes up the bulk of what is actually sold, and the two are worth keeping separate: quoting only the full range makes an ordinary pearl sound unusually large, and quoting only the typical range hides that larger examples genuinely exist and are priced accordingly.
Size is set mostly by which oyster or mussel species produced the pearl, since a small oyster cannot grow a large pearl regardless of how long it is cultured. That is why the four common cultured types have such different typical ranges: the animal, not the culturing time, is the ceiling.
The four common cultured types
Akoya, grown in Pinctada fucata off Japan and China, is the classic round white strand pearl and the smallest of the four common types — typically 6 to 8 mm, with an occasional example up to around 10 mm, because the oyster itself is comparatively small.
Freshwater pearls, grown in various mussel species mostly in Chinese lakes and rivers, typically run 6 to 9 mm but range more widely than any other type, from small seed-sized pearls up to 13 or 14 mm examples. One mussel can carry many pearls simultaneously rather than one at a time, which is the main reason freshwater is the most affordable type at any comparable size.
Tahitian pearls, from the black-lipped oyster Pinctada margaritifera around French Polynesia, typically run 9 to 13 mm and are the only common type that grows naturally dark — from silver-grey through green to a near-black often called "peacock" when it shows overtones of other colours.
South Sea pearls, from Pinctada maxima, the largest pearl oyster used in culturing, typically run 10 to 15 mm and produce the largest cultured pearls sold in any volume, occasionally reaching 18 to 20 mm. They are grown mainly around Australia, Indonesia and the Philippines and are priced accordingly — the same oyster that can grow that large also takes years longer to mature a pearl than an Akoya does.
Reading a strand by size
A strand described only by type tells you less than a strand described by both type and millimetre size, because the ranges genuinely overlap — a large Akoya and a small Tahitian can measure the same. If a listing gives only "Akoya pearl necklace" with no millimetre figure, that omission is itself informative: size is the detail sellers most want a buyer to ask about rather than volunteer, because it correlates so directly with price within a type.