The blogosphere slowly unveils the complicated truth. A reader writes:

The issue isn't how much gallium exists, it's how much can be recovered economically. Gallium is a trace element, so it can't be mined; it has to be extracted chemically from other minerals.

It exists in concentrations of only 50 parts per million in bauxite and zinc ores. So yes, there may be a global reserve of 1 million tons of gallium; but you might have to mine 20 billion tons (!) of other minerals to recover that. That's why only 100 tons of gallium is produced annually. Even that much required the extraction of millions of tons of ores. We're unlikely ever to run out of gallium, but as other minerals are depleted it will become increasingly expensive to extract and use. It's the same basic problem that we have with oil: there is plenty of the stuff left in the ground, but we are rapidly using up the portion of it that we can recover economically.

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