Just for fun I looked up quartz in Webster's unabridged. I turns out the ONLY
definition is the one for a mineral "a brilliant crystaline mineral". There is
no definition 2 that says "a high purity silica glass". However, a bit further
down we have "quartz lamp, a mercury vapor lamp having a tube made of quartz.."
I think Doug has a good analogy with wt given in kg. If enough people use
terminology loosly, it becomes part of the language and all precision in
language is lost.
It turns out that KY recently adopted coal as the state mineral. When it was
pointed out to the legislature that coal does not meet the accepted definition
of a mineral (which among other things must be inorganic in origin) the state
legislature was a bet perplexed. However, they had a good answer. We have
a minerals tax on coal so it must be a mineral!