China’s exports of rare-earth products jumped in June, pointing to a potential pickup in magnet supplies after government-imposed curbs that proved to be Beijing’s most powerful weapon in its trade war with the Trump administration.

Customs data released Friday show exports of all rare-earth products rose 80% from a five-year low in May, when the country was in the midst of implementing sweeping export controls. Magnets — a component central to recent trade tensions — typically form the bulk of the “products” category, but detailed export data on those won’t be available until Sunday at the earliest.

While volumes …