Is the practice of giving and eating clay as the Prasadam myth or not?

In Gujrat, there is the Dwarkadhish temple and adjacent to it there is a lake. Its soil turned into yellow clay that is famous as ‘Gopi Chandan’. Many Devotees use Gopi Chandan to put a mark (Tiak) on their forehead and some of them even eat it as they consider it auspicious.

There is interesting mythological story behind it:

Krishna and the Gopis were best friends. Once there was the full moon night and Gopis went to the lake near to Dwaraka. Krishna came to know about this by his divine power and reached there in a simple dress. He asked Gopis to return back to their village. Gopis denied and wanted Krishna to dance with them. 

Krishna had to bow down against the stubbornness of the Gopis and danced with them. After hours of wonderful dancing Krishna asked Gopis to swim in a nearby lake to relieve their fatigue. 

Gopis knew that they had to put away from Krishna after swimming, that they never wanted. All of them decided to end their lives where Krishna would not be with them. It is believed that Gopis were merged with their beloved and the clay of the lake turned yellow. 

Now, there are questions which usually devotees don’t ask and practice of giving and eating clay is going for generations over generations.

The questions are as follows that how they are sure that the particular incident turned the color of the clay yellow. The next question is that whether the clay which is given from the same auspicious spot where the incident had occurred or from somewhere else.