I picked up a stone looks like siltstone or claystone. but a geologist in our office said it's marl.

what's different between marl and those stones?

Marl is a rock containing clay and calcium carbonate. It is formed from the erosion of other rocks during weathering; as rocks erode, small sedimentary particles--sand, silt, and clay--pile up on top of each other. Eventually, these sedimentary particles become compacted together to form a new rock. The type of new rock that is formed depends on the original rock that was eroded and on the nature of the erosion. If the new rock contains predominantly clay and calcium carbonate, it is called marl.