Hi Steph,
All pearls that have been exported legally will have at least 0.8mm on the nucleus on each side. I say "legally" because the numbers don't add up down here. It would appear that there are large numbers of pearls making their way to Honk Kong that have never seen our x-ray machine. My site
www.kamokapearls has a pic of if you want to see it. This is all a bit frustrating for us farmers that are playing by the rules, x-raying all our pearls and paying the tax of $2.30USD a gram. Obviously the cheaters are not paying the tax either. I have even seen pearls in strands in websites mentioned on this forum that would not pass the initial visual inspection before the pearls get scanned. Sorry about the tyrade but sometimes it's hard to hold back on that one when it is constantly making things difficult for us fair-players. The cheaters can obviously sell lower and thus control more market shares.
To get back to your question, some pearls can even have up to 2 whole millimeters of thickness. Most are around or a little over 1mm.
Down here most farmers start with a 2.0bu (bu is Japanese measure that equates to about 3mm) which measures out to about 6mm. These are what you typically have in 8mm pearls.
The first graft operation usually spans between 2.0 and 2.9 bu with the majority for most farmers being 2.1 to 2.4.