Abstract: The "water line" inclusion is a typical appearance characteristic of "Laos stone" and "Shoushan Ducheng pit stone". The authors used macroscopic observation, gem microscope, polarizing microscope, X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and X-ray fluorescence spectrometer (EDS) to study the structure characteristics and chemical compositions of "water line" in "Laos stone" and "Shoushan Ducheng pit stone". The results showed that the mineral particles are larger in some layers of "water line" in "Laos stone" and have clear orientation, all of which are close packing flaky particles arranged in the direction paralleling to the "water line". Densely distributed dickite granules and the vein-like svanbergite compose the other layers of "water line" in "Laos stone". The "water line" and the substrate part of the "Shoushan Ducheng pit stone" define clearly. The size and density of the mineral particles between "water line" and substrate in "Shoushan Ducheng pit stone" do not change obviously. The finer silky svanbergite mixed between dickite particles compose the "water line" in "Shoushan Ducheng pit stone". The cause of the "water line" in "Laos stone" may relate to the multi-phase activities of ore-forming fluids and the differences in mineral composition. The cause of the "water line" in "Shoushan Ducheng pit stone" may relate to the different-period crystallization of ore-forming fluids and the differences in mineral composition.