Wingspan: 9–13 mm. Eriocrania sangii and Eriocrania semipurpurella are very similar and are difficult to distinguish from each other, genitalization is necessary to achieve species ID.

The young larva is light grey but it gets a distinctive dark lead-grey colouration on ageing.

The mine starts near a leaf margin with a short corridor and expands later on into a large, dirt-white blotch mine that continues in the direction of the main vein or even crosses it.
See also bladmineerders.be

The pupation takes place in a tough silken cocoon.

The egg is laid fairly close to the leaf edge. Therefore, the larva starts its mine also near a leaf margin with a short corridor and expands it later on into a large, dirt-white blotch mine that continues in the direction of the main vein or even crosses it. In the blotch mine the frass lies in long black threads. Mature larvae leave the mine and pupate in a tough silken cocoon in the soil.
The imagines can be observed resting on the twigs and leaves of the food plant or flying around these trees in sunshine. At night, they occasionally come to light.

The larva feeds monophagously on Betula spp.