The medium-sized, dark brown striped emeralds have some pale markings on the thorax and abdomen, black legs, clear wings and brilliant green eyes. The thorax has a metallic bronze-green sheen and the abdomen is dark metallic black-green. Most species are uncommonly seen, but this may be largely due to their secretive nature. When flying, most species look alike which is why in-hand identifications of their anatomical features are often needed.
This is a dark striped emerald that has obscure markings at maturity. The length of the body varies from 2.0 to 2.3 inches. In the young adult stages, the Williamson's emerald is similar to the ski-tipped emerald. As the Williamson's emerald matures, the lateral thoracic spots fade, making these two species easier to tell apart. The females are similar to clamp-tipped emeralds' females.
Common throughout southeastern Canada and northeastern United States, including the Appalachian Mountains, it is usually found at slow streams and lakes, and sometimes bog lakes. It seems to prefer shaded habitats. This species is fairly widespread throughout much of Wisconsin.
Late June to early September in Wisconsin.