What is this 3 cm brown seed?

Question

I found this seed on the beach on Koh Jum, an island on the Andaman Sea. The islands nearby are mostly populated by black mangroves (Avicennia germinans) and rubber trees (Hevea brasiliensis).

Answer 1

It's definitely a sea bean - as to which, most likely because of its vaguely 'hamburger' appearance (apart from reversing the colours) it's Dioclea reflexa, usually grows in rainforests, but its seeds wash up on tropical beaches all over the world. The link below contains some interesting information and illustrations regarding all kinds of sea beans. If you want to find this particular seed, scroll down to the end - there's a small photograph insert on the right showing various seabeans with ID letters and plant names beneath.

http://waynesword.palomar.edu/mucuna.htm

Answer 2

After doing some research it appears to be some sort of sea bean. From Wikipedia:

[…] common names like deer-eye beans, ox-eye beans or hamburger seed.

This Google image search confirms that.