Basalt, an igneous rock formed by the heat of volcanoes, has a distinct hexagonal structure.
This informational sign from the Columbia Gorge in Oregon explains the volcanic process in which basalt is formed. I have kind of messed the image up by collaging it but you get the idea, no? 
Latourell Falls, one of the many waterfalls along Historic Hwy 30 in the Columbia Gorge, with basalt rock formations behind it. 
Devils Postpile in Mammoth, California.
Giants Causeway in Northern Ireland, UK showing the distinct hexagonal formation of the volcanic rock basalt.
More of the striking hexagonal basalt rock formations of Giant’s Causeway in Northern Ireland.
My 4 foot by 8 foot painting of basalt.
More of Nancy Merrill’s Photo a Week Challenge: Structure in Nature.
Batz Gallery 
So cool! The sign about how it forms is fascinating. I’ve seen the same type of formations on Staffa Island in the Inner Hebrides, Scotland. Hope to make it to the sister island in Ireland soon. 🙂 Thanks for joining the challenge, again!