In the bracha to Yissachar and Zevulun Moshe mentions that
For by the riches of the sea they will be nourished, and by the treasures concealed in the sand.” (Devarim 33:18-19)
Rashi explains that the
"concealed treasures of the sand” refers to “white glass” that is found in the sand.
What is the significance of this white glass? It does not seem to serve any mitzvah purpose, as opposed to the techeles that is also part of the bracha of Zevulun. There is no indication that it was a rare and valuable commodity (like precious metals/stones). So what significance does it have?