Oysters make them. Once in a while, an oyster gets what we might think of as a splinter—some little bit somehow gets in between the oyster's shell and its body (specifically, its mantle). This little bit of grit (or sand or bone or what have you) is very irritating to the poor oyster—but the oyster doesn't have hands and tweezers to get the thing out! All it can do is use its shell-making apparatus to cover up the irritating bit. The layers of shell (nacre) covering the “splinter” are what we call a pearl.
And in the wild, they are very rare.
Even more rare are the beautiful spherical pearls. Some pearls turn out to be oval or even knobbly. They are called baroque pearls.
This rarity means that spherical pearls harvested from wild oysters are very, very expensive!
The second way of making inexpensive pearls is (of course) to make fakes! “Faux” or imitation pearls are made in several different ways, but on this date in 1656 a French man named Jacquin patented a way of manufacturing imitation pearls from blown-glass spheres coated on the inside with ground fish scales.
Real pearls are iridescent, which means that they shine with different colors depending on the angle of the light. Other things that are iridescent include the insides of sea shells (which are made of the same substance as pearls), butterfly wings, soap bubbles, and fish scales.
Jacquin noticed the iridescence of fish scales and ground up some scales from a fish called bleak (or ablette) and then applied the scales in a liquid form to the inside of glass spheres. The hollow balls were then filled with wax to make the weight similar to that of a natural pearl.
There have been many changes to manufacturing imitation pearls since the 17th Century, of course, but many faux pearls are still coated glass or acrylic beads.