Here's a short guide.
Real pearls are produced by certain species of living, shelled mollusc. The finest quality pearls are highly valued as “organic” gemstones because they have
- lustre (soft pearly sheen) and
- iridescence (colour changes with the angle of viewing)
Incidentally, these qualities are due to layer of nacre. Nacre is a special type of shell lining and can itself be used as jewellery, usually called mother of pearl.
How are Natural Pearls Made?
When a pearl is formed in the wild it is because an irritant e.g. a parasite or other organic particle gets inside the shell (it's hardly ever a grain of sand). The natural defence mechanisms of the mollusc cover the invader in order to seal it off and protect itself. These protective layers are made of nacre and it takes many years to build a pearl as each millimetre of nacre takes about three years.
How are Cultured/Cultivated/Farmed Pearls Made?
When a pearl is farmed or cultivated, the irritant is introduced on purpose, and it tends to be much larger to begin with- typically a mother of pearl bead is used and then the mollusc deposit a few layers of nacre over a minimum period of several months.
How are Crystal or Swarovski Pearls Made?
Crystal pearls are manufactured, usually with crystal core and then a special pearl coating designed to mimic the soft sheen of a natural pearl. Swarovski pearls have a very pleasing weight and good lustre and are very hard-wearing. Crystal pearls tend to be much harder-wearing that organic pearls which can be damaged by perfume or even sweat.