USGS · 70003937
Although a relative youngster in geologic terms, obsidian can change over time from glass to mineral.
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W. H. Langer. 2011. O is for obsidian. https://pubs.usgs.gov/publication/70003937
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U.S. production of construction aggregates in 2011 was 2.17 billion short tons, valued at $17.2 billion, free on board (f.o.b.) at plant. Construction aggregates production decreased by 37 percent, and the associated value decreased by 25 percent, compared with the record highs reported in 2006. In 2011, construction aggregates production increased for the first time since 2006, owing to a very slight increase in the production of both construction sand and gravel and crushed stone. The average unit value, which is the f.o.b. at plant price of a ton of material, increased slightly, but is still less than the average unit value of two years prior.
In 2010, the market appeared to bottom out with production continuing its downward trend and average unit values down for the first time since 1998.