Glaucomaria rupicola

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.