Thanks for the information. There's so much contradictory stuff on the internet. This helps loads!
It would first make a difference if you are looking for a pearl that is 10-11 mm or 11-12 mm. Even if the pearl is 10.95 mm, it is going to come from a smaller lot than say a pearl that is 11.05 mm, so there would be a noticeable difference in cost.
Tahitian pearls are available in nearly every color under the rainbow, from white to dark black.
The best quality would be a pearl that is round, with sharp luster, and no surface blemishes whatsoever. To most people in the trade, a pearl with one small spot is just as valuable as a pearl with no blemishes, because that spot is known as the drill spot. But some traders do pay more for the top gem without even a single spot.
Thanks for your reply! If comparing two pearls, both equally round with equally sharp luster, same size (let's say 11 mm) but with different colours, how big difference will it be in price just because of the colour? Could a "rare" colour mean that the pearl cost 100% more than the pearl with the more "common" colour?