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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?

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It was expensive - see the beginning of the fifth perek of brachos – Shmuel Brin Aug 12 '12 at 18:49
@ShmuelBrin can you give a more exact source? – not-allowed to change my name Aug 12 '12 at 20:17
The top of the first full blatt of the Masechta has an Amora breaking a cup of white glass to stop leitzanus – Shmuel Brin Aug 12 '12 at 20:28
See Brachos 31b where Rashi interprets an expensive cup as a white glass cup and so too I'm brachos 28a – Shmuel Brin Aug 12 '12 at 20:47
An important point, is that glass being a clear see-through substance which you can use for windows and can barely see, is a relatively recent invention. You can see glass from the 1800s which is a mirky white used for windows. – avi Sep 30 '12 at 7:20
up vote 6 down vote accepted

One possibility: it would seem that when the art of glassblowing was still new finely made glass was considered very precious. The pasuk in Iyov (28:17) equates glass with gold

לֹא-יַעַרְכֶנָּה זָהָב, וּזְכוֹכִית; וּתְמוּרָתָהּ כְּלִי-פָז - Gold and glass cannot equal it; neither shall the exchange thereof be vessels of fine gold. Also, note per the mishna in Sota that after the destruction of the second temple "white glass" is no longer extant.

cf: Hidden Light: Science Secrets of the Bible (pgs 111-12) - By David Medved

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+1 Metzudot Tziyon on that pasuk explicitly says the referenced glass is white glass. – Double AA Aug 13 '12 at 1:56
Wikipedia talks about the glass trade of the region during the Late Bronze Age: en.wikipedia.org/wiki/… – Charles Koppelman Aug 13 '12 at 14:52
Can you cite the Mishna in Sotah? – Double AA Sep 30 '12 at 3:46

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