At first glance: It looks like a cultured pearl to me.
It's very nice!
Remember that cultured pearls make up about 99% of all pearls on sale...so, just with that in mind it would be rare if you have a natural pearl.
Hi! I want to sell it but I don't have any evidence that it's a natural pearl. They're looking for a certificate but I think there is no laboratory for pearls here in the Philippines. Can I have some advice on what should I do or where can I get a certificate for my pearl in the Philippines?
I would send it over to the Bangkok GIA lab. It's the closest you have for this service.
BUT before you do: try to find a local dentist with good x-ray equipment. Bring 2 pearls, one cultured (that you fully know is cultured) and this pearl you want to analyze, and have them x-rayed.
This is how cultured pearls would look like using a Dentist's x-ray apparatus. All these pearls were analyzed because they were supposed to be "natural pearls". You can clearly see the bead inside.