Columnar Jointing

When a thick sheet of lava cools, it contracts. Contraction pulls the rock apart, and the cracks that open propagate down into the flow as the cooling front advances, leaving a set of roughly parallel columns.

#Why hexagons

Cracks form where tension is highest and propagate to relieve the most stress for the least new surface. On a plane, the arrangement that divides area into equal cells with the least total perimeter is a hexagonal tiling, so a cooling sheet that cracks efficiently tends toward hexagons.

Real columns are not perfect. Counts of four, five and seven sides are common, and the average sits near six rather than landing on it exactly, because cooling is never perfectly uniform.

#The same reason H3 uses them

A hexagonal grid gives every cell the same number of equidistant neighbours, which is why hexagons keep turning up wherever something has to tile a plane fairly — basalt and spatial indexes arriving at the same answer from different directions.

See Basalt for the material and Giant's Causeway for the type locality.