A key part of the attractiveness of the Blue lagoon and Myvatn Nature Baths in Iceland is the tremendously attractive but rather unnatural blue color of the water in both lagoons. But why is it blue and not gray, silver or red for that matter?
The reason is pretty simple. The water in both lagoons flows from nearby geothermal plants using underground water to power equipment to produce electricity. Since Earth´s crust is in large part made from silica a lot of that gets also pumped up to the surface with the water. Afterwards the waste water from the plants is pumped out to the nearest field.
Now silica is a pretty sticky substance, as evidenced by silicon for example, which is why when it cools down it gets stickier still and quickly forms pools of water or lagoons if conditions are right. The lava fields of both Reykjanes peninsula where the Blue lagoon is located and Namaskard of Myvatn where the Nature Baths are found make for excellent spots in this regard. The silica binds with the porous lava and closes it up in time forming a pool.
Silica molecules, which content in the lagoons is 140 mg/kg, radiate more blue in daylight than other colors and that fact gives the lagoons their distinctive color.