What inspired me?
A boulder on a south coast beach.
 
I loved the contrasts of plain surface, washed smooth by countless waves, against shining seams of what was probably quartz. I know nothing about geology, so that's my best guess. (Do get in touch if you have a better suggestion.)
 
Smoky grey pigment, reminiscent of a watercolour wash, and small bursts of ochre completed the design.
 
I considered making an undulating surface, to mimic the boulder, but refrained as I felt that would just have detracted from the design.