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A medium-size moth (4.0-4.5 cm wingspan). Forewings are dark shining grey with strong violet tones. The median area of the forewings below the stigma is a darker rusty black, often with a small rust patch along the lower postmedian line. The prominent stigma is an outward curving silver arc, with the upper half forked. Hindwings are smoky brown, darkening into a broad band along the outer margin. The similar A. california is a bit lighter and less purple, and can be separated from pseudogamma by the short, fine black dash connecting the subterminal and postmedian lines near the apex of the forewings.

Newfoundland to coastal northern Alaska, south in the east to New England and in the western mountains to New Mexico, Arizona and California. Also found in the Cypress Hills and The Black Hills of South Dakota. In Alberta, it can be found throughout the Mountain and Foothills regions as well as along the southern part of the Boreal Forest; also in the Cypress Hills.