[3] Species in this genus typically grow on nutrient-rich bark or somewhat siliceous or calcareous rocks in humid environments.

The lobes are 2 to 15 mm broad, rounded, and usually flat or partially raised, with smooth or pustulate surfaces adorned with elongated ridges or folds.

[4] Both upper and lower cortex are absent in Collema, and the photobiont Nostoc forms chains of cells throughout the thallus without creating a separate layer.

The epithecium is indistinct and can be colourless or pigmented, while the hymenium is colorless and turns blue when treated with iodine.

The hamathecium comprises paraphyses that separate in a solution of potassium hydroxide and may be unbranched or branched, often connecting near their tips, which can be club-shaped or round and exhibit a yellowish to reddish-brown hue.