There are two basic types of farmed pearls: bead nucleated and tissue nucleated. (The other main type classification is between cultured or farmed and natural or wild pearls)
Nucleation is the process which starts off the growth of a cultured or farmed pearl. It involves inserting something into a nacre-producing mollusc to trigger production of a pearl. This nucleus can be either just a tiny sliver of mantle tissue on its own or a sliver of mantle tissue plus a bead or other shaped foundation. In either case a nacre secreting pearl sac grows and a pearl is made within that sac.
Bead nucleated pearls include all tahitian and south sea pearls, akoya pearls and many modern big freshwater pearls (brands Edison and Ming) as well as fancy shapes such as coins or hearts.
Tissue nucleated pearls are mostly all freshwater pearls which are therefore all nacre, solid pearl. no bead inside. (Chinese and Biwa freshwaterpearls)
Keishi pearls are an exception. They are the pearls formed inside a usually pre-existing pearl sac from which a pearl has been removed (think of how a balloon looks when the air seeps out over time and you get the idea of a keishi pearl.
Mantle tissue is used because that is the area of tissue which specifically secretes nacre. It’s usually there to make the mollusc’s shell but will produce nacre wherever it is – a talent utilised by the pearl farmer.
Placement of the nucleus varies as well. Beads are placed in the sex organ – the gonad – of the mollusc and only one per mollusc. (You might think that this would stop the mollusc from wanting to reproduce but there is some research which indicates it make them more not less active!)
Tissue nucleated pearl grafts can be many to a mollusc and are placed in the mantle.
The new big thing (last three or so years only) in pearls are the big freshwater bead nucleated pearls from China. These are branded as Edison pearls or Ming pearls or are described as ripple pearls. They are big – up to 17mm, round or symmetrically baroque (because of the bead inside), have a lustre ranging from metallic to gloriously satiny, and a smooth to convoluted or rippled. They come in a range of natural colours from white to pink, purple, peach, gold, with greens and blues. (you can see our ripple pearls here and we have smooth surfaced Edison and generic bead nucleated pearls throughout the freshwater section ..here is one beautiful example of a bead nucleated Edison pearl necklace