I agree with Fred -we see this quite often with porous stones.
Just to add-
It is hard to solve as I think Fred will agree that stone is possibly damp stained.
Damp stained stone is very hard to correct 

On Thu, Jul 19, 2018 at 11:47 AM, Fred Hueston <fhueston@gmail.com> wrote:
this is a common problem in showers. It may be the shower pan is backing up with water. I would first check the weep holes in the drain. If there clogged the shower pan wont drain properly

2018-07-19 11:44 GMT-04:00 Stuart Young <santafefc@sbcglobal.net>:

Recently installed marble shower. Lower rung of tiles darkening (getting wet). Any solution?
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Frederick M. Hueston
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Stu Rosen
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