[xoxoxoBruce]

The walls are covered with psychedelic patterns, caused by the natural layers of mineral carnallite creating swirls throughout the coloured rock. Carnallite is used in the process of plant fertilisation, and is most often yellow to white or reddish, but can sometimes be blue or even completely colourless. The mineral carnallite, a hydrated potassium magnesium chloride, decorates the empty tunnels under Yekaterinburg.

And Wiki says;

Carnallite occurs with a sequence of potassium and magnesium evaporite minerals: sylvite, kainite, picromerite, polyhalite, and kieserite. Carnallite is an uncommon double chloride mineral that only forms under specific environmental conditions in an evaporating sea or sedimentary basin. It is mined for both potassium and magnesium and occurs in the evaporite deposits of Carlsbad, New Mexico; the Paradox Basin in Colorado and Utah; Stassfurt, Germany; the Perm Basin, Russia; and the Williston Basin in Saskatchewan, Canada.

Remember, the more you know, the less your sure of.