Just by lake has left stone (4)
I believe the answer is:
onyx
'stone' is the definition.
(type of gemstone)
I don't understand how the rest of the clue works.
(Other definitions for onyx that I've seen before include "African antelope" , "Form of agate" , "Black variety of quartz" , "Marbled stone with layers of colours" , "Banded chalcedony" .)