Resource consumption is set to rise in coming decades. A circular economy can reduce environmental impacts by cutting raw material use and emissions but will still require some primary resources due to growth, stock expansion, and dissipation. The planetary boundary framework could help set limits, yet it currently lacks boundaries for minerals, metals, and fossil resources. This study proposes a principle for such a boundary, based on net accessibility and fair resource allocation across generations, to define a sustainable extraction rate for a circular economy. Further work is needed to operationalize this approach.