a. Hardened in wrongdoing or wickedness; stubbornly impenitent: “obdurate conscience of the old sinner” (Sir Walter Scott).
b. Hardened against feeling; hardhearted: an obdurate miser.
2. Not giving in to persuasion; intractable. See Synonyms at inflexible.
On a more positive note, the phrase “apple polisher” describes a positive attribute and yet is a bit of a slur that seem to demean both the person and the wholesome apple at the same time.
Pliny’s Natural History [published 77 AD] features volcanic glass called “Obsidianus”, so named from its resemblance to a stone found in Ethiopia by one Obsius.