What do I do with the gems in the lava room of Sky Keep?

A sign next to the exit tells you what you need to do.

Seek the gemstones that sleep behind each statue. If you strike them in order from lowest to highest, the door will open.

After uncovering all three switches, simply strike the switches in order based on their elevation within the room, from lowest to highest, in order to progress.

To be exact, the correct order is:

• The one at the bottom of the sand slope

• The one near to the entrance

• The one behind the thin trees

_{Quote retrieved from: http://www.zeldadungeon.net/Resources/Textual-Resources/Zelda14-skyward-sword-text-dump.php}

According to this walkthrough for Sky Keep:

The key to solving this puzzle is that you have to hit the diamond switches in a proper order. The correct order is 2, 1, and then 3. So Use the beetle to hit the diamond at the northwest part of the room, then the southwest part of the room, and finally the one at the northeast part of the room. Hitting them in the proper order will cause a nearby gate to open up. Jump on down and head through.

This is based on:

The height of the diamonds - this other walkthrough explains: "In this room there are three pink switches and they must be hit in a specific order from the lowest to the highest."

So:

Hit the diamonds in the proper order (bottom to top; lowest to highest) and it should work!