IIRC this is not because Iceland has particularly rich stores of aluminum ore, but rather because refining aluminum ore into usable aluminum metal is energy-intensive and Iceland has abundant geothermal energy resources. So they import ore and export metal.

It's actually fish, aluminum being a close second:

http://px.hagstofa.is/pxen/pxweb/en/Efnahagur/Efnahagur__uta...

To answer child, the type of electricity production is split 70 hydro/30 geothermal.

Depending on what you measure the largest export of a single product is raw aluminum but industry wise fishing products as a whole are still a larger proportion. Also depending on how you want to classify it both are eclipsed by tourism right now.

They have a big mayonnaise industry too, or did when I was there 15 years ago