What is usually used as the nucleus for a cultured pearl?

[Professor (male)] Good morning, class. Let’s get started. Q6 Uh, what I wanted to talk with you
   about today is the formation of pearls. More specifically, I’d like to compare two
   ways that pearls form: naturally...and with human help. Let’s start by talking
   about how pearls form naturally. I suppose most of you already have an idea
   how pearls develop in oysters, but let’s just go over the process to clear up any
   misconceptions you may have and make sure we’re all working with the same
   information. OK? Natural pearls begin with a nucleus.
[Student A (male)] You mean like a grain of sand or something? I’ve heard that pearls form around
   a tiny grain of sand that gets into an oyster’s shell. Is that true?
[Professor] Well, the nucleus could be a grain of sand...a tiny parasite...a particle of plant
   material--just something that finds its way into an oyster’s shell when it’s
   open for the purpose of feeding or respiration. But the thing is...this nucleus,
   whatever it is, it irritates the oyster. When this foreign particle becomes lodged
   between the oyster’s shell and its mantle--urn, which is the membrane that
   surrounds its soft body--when this happens, the oyster responds to protect
   itself from the particle. Q7 So what it does is coat the particle in nacre. And
   what’s nacre? Nacre is that calcium carbonate material that covers the inside of
   an oyster shell. You know...the shiny stuff. Over and over, the original nucleus
   gets covered in layers of nacre. And when these layers build up enough, they
   form what we recognize as a pearl. Q8 If you looked at a cross section of a
   natural pearl, you’d see a series of concentric circles, like the growth rings of a
   tree, And it all started with that tiny foreign particle that got inside the oyster’s
   shell.
[Student B (female)] So...natural pearls are formed just by accident, really? I mean, the oyster doesn’t
   intentionally create the pearl for any reason?
[Professor] That’s right. Pretty interesting, huh?The creation of natural pearls is just a
   response to an irritating particle. Um...also, since they form under such specific
   circumstances, natural pearls are very rare, as you can imagine. Q9 But what’s
   even rarer is a natural pearl that’s perfectly round. And since a perfectly round
   pearl is nicer to look at than an irregular one, um...round natural pearls tend to
   be really expensive.
[Student B] Q11 But nowadays pearls are more affordable in general...because
   we have the ability to manufacture them, right? And it’s easy to make them
   perfectly round.
[Professor] Well, yes. Pearls that form with help from humans are known as cultured pearls.
   But, uh, manufacture probably isn’t the right term. It’s not like we’re
   building them in a factory. Cultured. pearls form in much the same way as
   natural pearls--I mean, we still rely on oysters to create them.
[Student A] So how does the process work...the process of making a cultured pearl? Do
   people actually, um, insert nuclei into oysters?
[Professor] That’s right. The technology to create cultured pearls was developed during
   the beginning of the twentieth century. And it basically works just like you
   described it. Q10 A nucleus is surgically implanted inside an oyster’s shell.
   Um, this nucleus...they’re usually made from a piece of the shell of an oyster...
   or another shelled mollusk. And...well, once the nucleus is in, the oyster pretty
   much takes over from there.The process is essentially the same as natural pearl
   formation. Nacre coats the nucleus and a pearl forms.
   Uh, aside from the fact that cultured pearl formation involves humans inserting
   a nucleus, the main difference between natural and cultured pearl formation
   is the size of the nucleus. Natural pearls begin with a rather tiny particle, and
   the bulk of the pearl is made of nacre--layers and layers of it. Well, in cultured
   pearls the opposite is true. Most of a cultured pearl’s mass is made up of the
   artificial nucleus, and the nacre coating on the outside is just a thin layer.
   Because of this, the nuclei of cultured pearls are really more like beads than tiny
   particles. They’re nearly the same size and shape as the final pearl. Q8 If you
   looked at a cross section of a cultured pearl, it’d seem pretty different from the
   cross section of a natural pearl. As I said before, natural pearls have tiny growth
   rings. You know, just like a tree has growth rings every year as it gets bigger, or
   uh, shells get growth rings as they develop. But, uh, if you looked at the cross
   section of a cultured pearl, you wouldn’t see any growth rings, just a thin layer
   around the nucleus. Most of the interior would just be solid nucleus bead. Got it?