Literally 'sheet-hemp-rock': 片 'sheet/slab' + 麻 'hemp' + 岩 '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 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.
