The joke is a pun.  'Gneiss' is the name of another type of metamorphic rock.  'Gneiss' sounds like 'nice', so in a sentence it sounds like the geologist is complimented the craftsmanship of the sculpture, but they're actually saying it's a different type of rock.
Upon being shown a uniquely sculptured piece of so - called granite , a geologist remarked to his colleague ' ' You know that ' s really gneiss . ' '