You can read the guide about Iriomote made by sanji :
Iriomote (secret-japan)
And if you know about google map try to paste this : 24.436894,123.77738 in map.google.co.jp
This is where I found my star sand in Iriomote (same beach as the one describe by sanji)
Hope this will help you
EDIT : oups sorry, thought you were going to Okinawa take your own. I cannot help with the post.