Selenium is the most extreme byproduct story in the atlas: it has no primary mines and no separate reserve category — its entire commercial supply is a function of electrolytic copper (and to a far lesser extent lead-zinc) refining throughput. The supply is geographically dispersed compared with the extreme concentration seen in DRC cobalt or Chinese rare earths, but it is structurally captive: selenium output cannot be scaled up independently of the copper refinery cycle. China now accounts for roughly 49% of world refinery output, reflecting a decade of copper refining capacity expansion (~60% increase in electrolytic copper capacity, ~doubling of copper anode capacity per USGS). The US produces crude selenium and anode slimes at two facilities but does not domestically refine selenium; all US consumption relies on imported refined selenium, consistent with net import reliance reported as >50% across 2020–2024.

The 2024 price uptick (US $23.11 → $24/kg, Europe $19.30 → $24/kg) is notable after the relatively stable 2022–2023 period; USGS attributes the European rise to limited availability and higher costs of purchasing from China. Peru's refinery output collapsed from 850 t (2023) to 50 t (2024) with no explanation in the USGS chapter — this is a significant data anomaly worth monitoring.

End-use structure is shifting: metallurgy (electrolytic Mn, steel) at 40% is the largest single sector globally, but Chinese demand in this segment has been declining while electronics and solar (CIGS) and agriculture have grown. The long-anticipated CIGS photovoltaic demand driver has not materialized at scale relative to crystalline silicon or CdTe competitors; amorphous silicon and CdTe remain the principal thin-film alternatives.

Selenium on the US critical minerals list: inclusion in USGS MCS 2025 is the operative designation; the USGS chapter text does not state this explicitly, and an EU-specific citation would be needed to confirm EU CRM Act 2024 status.