Update: Here's a painting (which turned into a diptych in the process) that is an attempt to capture the painting in the dream.
My favorite dream in awhile: A lone painting in a frame hangs on a wall in a dimly lit, unfurnished room. I cannot identify the subject of this work of art but I am told by the man standing nearby that it is by a "master artist". The painting grows a chalky film in patches. (The film is a little like the waxy "bloom" that can occur with the build-up of certain media, e.g., colored pencils or oil pastels.) The chalky film spreads across the painting and sprouts what appear to be tiny quartz crystals and these, in turn, grow to become precious and semi-precious gemstones. The man takes a knife and gently scrapes at the painting, collecting the gems into a cup. The painting grows dark once more before the chalky film reappears...