Rather easily recognisable by the yellowish triangular spot on the forewing costa, the outer margin of which is prolonged towards the wing apex. In the similar species Caloptilia robustella, this spot is not prolonged towards the apex.
Other differences: crest on C. alchimiella purple-brown, darker than the thorax; in Caloptilia robustella yellowish as the thorax.
Thorax in C. alchimiella gold-yellow with pale purple-brown tegulae; in Caloptilia robustella yellowish with same-coloured tegulae.
Forewings of C. alchimiella with sharply defined basal spot; limited in Caloptilia robustella and diffuse.
External border of the costal spot in C. alchimiella almost ending in the apex; in Caloptilia robustella at about five-eighths striking steeply on the costa.
Small and indistinct yellow spot on the tornus (inner angle) present in C. alchimiella, absent in Caloptilia robustella.

Underside gallery mine on Quercus leaves.
See also gracillariidae.net and bladmineerders.be.

Eggs are laid on the underside of a leaf. The caterpillar, when young, forms a gallery mine at the underside of a leaf, later it lives in 2 or 3 successive cones, rolled down at the top of a lobe—pupation under a greenish membrane on the underside of a leaf. The pupa hibernates.
The adults rest on tree trunks during daytime and later come to light.

The caterpillars feed on different species of Quercus, but mainly on Quercus robur. Occasionally also found on Fagus sylvatica.

In most of the localities where Quercus trees grow.