rock, crag, boulder
巌 centers on large, rugged rock formations: a massive boulder, a steep crag, or a towering cliff face.
巌 is the older form of 厳, with 山 added on top to emphasize the rocky, mountainous sense. The lower part 厳 originally meant steep or severe, and the addition of 山 narrowed the meaning to a steep rock or crag.
A mountain 山 sits on top of a severe, imposing shape 厳, like a giant boulder perched on a steep cliff.
For ガン, picture a gun (ガン) being fired at a huge rock, but the bullet just bounces off the crag.
huge rock; massive boulder
near-vertical cliff or peak
craggy; steep (as of a mountain)
The Count of Monte Cristo (novel by Alexandre Dumas)
withered trees and frozen rocks; a person that is hard to approach
Blue Cliff Record; Biyan Lu (collection of Zen Buddhist koans compiled in China during the Song Dynasty)