Read "Symbols" at The Eldritch Dark:
The version of this poem at The Eldritch Dark has a small typo in the second line: the mis-spelled word "vemilion" should be "vermilion."
With the corrected text, we have a powerful description of the artist and his muse(s), and it's hard to read these lines as anything other than a personal creative statement from Clark Ashton Smith (CAS). His imagination is fired not by "gold and marble," but by things of a darker strain:
To body forth my fantasies, and show
Communicable mystery, I would find,
In adamantine darkness of the earth,
Metals of any sun; and bring
Black azures of the nether sea to birth—
Or fetch the secret, splendid leaves, and blind
Blue lilies of an Atlantean spring.
The phrase "Communicable mystery" has an echo of Arthur Machen's interest in the quality of "ecstasy" in literature, as detailed in his book Hieroglyphics (1902), a volume that CAS admired*.
*See letter #208 in The Shadow of the Unattained: The Letters of George Sterling and Clark Ashton Smith published by Hippocampus Press.