Concretion｜結核：圓石不一定是被水磨圓，礦物膠結也能從核心向外長成球
WHAT 這是什麼
Concretions form when mineral-rich groundwater precipitates cement in pore spaces around a nucleus within sediment or sedimentary rock. Growth can proceed outward from that center and produce rounded masses, including the large cannonball concretions described by the National Park Service.
WHY HERE 為什麼屬於底域的世界
它拆掉「圓＝被河水磨過」的單一路徑：圓度也可以是礦物在沉積物內部向外膠結、生長的結果。
BLIND SPOT 補了哪一塊少看的視角
結核可呈球形、扁圓、不規則形，礦物組成也很多樣；僅憑圓形外觀不能判定它是化石蛋、人工球或特定結核類型。
