Answer
Sandalwood - You need to have the metal hammer (made with the forge or something that is in the temple on the third island). The only tree is by the three hot springs
Metal Ore - Right at the very top of the volcano!
Glass - At the forge of the temple. You need either fine sand which is found on crystal island which you must unlock, or those bottles you get messages in.
Now for your main question - you need to have been on island three to get through that door. If you do not want a spoiler, then stop reading :D
On island three you find all of those hieroglyphics. Using your workbench you make them into one nice big one. You can then enter the two ancient doors on island three. Inside there is a crown near the two ruined seats. THAT is the key to the second island's door!
It's worth it to get in that door - it's a pretty site :D
Also in case you didnt know there is on the second island a rock on west beach near the coldesack with pineapples. Inspect it and oyur sim will move it out of the way. There is a hidden passage way wich leads to Pirates Cove and is wear Beard Beard's treasure is.
Actually there are several sandalwood trees around that hot springs and also about three of them in the geyser plains area. I am not sure about any other locations, but those I do know of.