You pointed out one of your pearls was a tear drop.
Put it in the palm of your hand. Look closely at the pointier end as you slowly turn it.
As you turn, focus on the middle pointier bit. Don't look at the edges as they will change as they reflect your skin.
Once you have focused on the nacre and colour, start to move down to the boulbus end doing the same inspection until you have finnished at the bottom.
All nuclei in South Sea Pearls are round, with a teardrop shape the nuclei will be in the boulbus section.
If you find an area that differs in colour, it's pretty likely a thin area of nacre.
Two things I personally recommend when buying pearls.
1: Roll them in your hand to SEE them. If the vendor won't let you touch it, walk out.
2: Understand that if the pearl you are buying is already mounted as jewellery, you will never know if it was a gem grade or not. Jewellers love to buy B grades, cover the flaws with a mount and sell them as Gem grade.
If you are buying over the internet, do back ground checks on the seller.
I hope that explains it. Have a nice day.