White Rim Lichen

Glaucomaria rupicola, the white rim lichen, is a species of crustose lichen in the family Lecanoraceae. It forms thick, whitish-grey patches on hard, exposed siliceous rock such as granite, from coastal cliffs to mountain summits. The fruiting bodies are typically covered by a conspicuous pale grey powdery coating, and the disc reacts bright yellow with bleach (the C spot test). The species has a wide distribution, occurring across Europe, North America, and Asia in the Northern Hemisphere, and extending into South America and other southern regions. It is particularly common on wave-splashed rocks just above the high-tide line and on well-lit rock outcrops inland. A 2025 molecular study confirmed its placement in the resurrected genus Glaucomaria, separate from Lecanora in the strict sense.

Excerpt from the Wikipedia article Glaucomaria rupicola, by its authors, under the CC BY-SA 4.0 licence.