Literally 'sheet-hemp-rock': 片 (piàn) 'sheet/slab' + 麻 (má) 'hemp' + 岩 (yán) 'rock.' The 'hemp' part captures the woven, fibrous, banded look of gneiss layers — exactly the texture geologists call 'gneissic banding' in English.
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Earth's oldest known rocks — the Acasta Gneiss in Canada and the 阜平群 (Fùpíng Qún) gneisses in Hebei — are over 3.8 billion years old. Gneiss is what you get when granite or sedimentary rock is cooked and squeezed deep in the crust.
