Magic comes from shapes. For example, when you inscribe a regular octagram in a circle you get a heat effect, and the right straight line atop that summons a creature from beyond the mortal plane. Human interaction is not even required: the shapes themselves resolve into various magical effects. The laws of magi-physics just work that way.
That has some consequences. Patterns come about from mere chance, and in my world they produce magic. Piles of branches are found in the forest floor, and every now and then a subset of them might be in the right shape to produce a spell. There are many trees and spells may live for a long time, so due to the trees serving as infinite monkeys on typewriters, the forest is a place where you stand a decent chance of finding "wild magic".
Anthills and mole tunnels are other examples. There's less of them than forest branches but still you might find an unlucky subterranean mammal that dug a magic shape and had their home filled with rice pudding.
My question: where else may you find wild magic? Where might you find complex geometric shapes in nature, taking infinite monkeys into account?
- A geometric shape is composed of (rough) lines made of one material, in a medium of one or more different materials. A bag of sand doesn't have any shapes because one particle isn't distinct from the rest; waterflows with varying salinity on the ocean floor also won't do. Marbling is barely enough to produce something measurable, and it won't summon creatures. The more distinct the materials the more potent the effect.
- Stuff like patterns in sunflower seeds doesn't count, because the seeds define the vertices of a shape, and magic geometry is defined with the edges.
- Disregard human activity. People are careful when bundling their firewood and have even begun laying out their village roads to form protection spells - but that is outside the question scope. This is only about "wild magic".
- Assume regular earthly flora, fauna, and physics/geology. This effect has not been around for long enough to affect evolutionary development. The exception is that web-making spiders have gone extinct, for obvious reasons (many inscribed circles make an electricity spell). Bees and wasps lucked out on this; tessellated hexagons are too simple to make magic happen.
- For the kinds of shapes that are magical, consider geometric mandala shapes (1, 2, 3, 4) - and certain subsets of those (like a pacman or a bowtie shape) and/or extensions (some extra lines atop). A bit complex for a random pile of branches, but again, infinite monkeys.
- When a shape is more than half-way reached, the laws of probability bend and any remaining random contributions to the shape are much more likely to end up in the right locations. Nature wants magic shapes to happen.
- Minimum size is 1 cm in diameter. Otherwise snowflakes would be a major issue. Furthermore, magic potency scales with physical size too (though less so than it does with medium contrast).