GlassI always laugh when you refer to me as glass.
I will be critiquing this piece on behalf of
Firstly, I love the way you draw the reader in with your first ST by explaining the 'glass-as-in-gas' pronunciation.
Now, the crit:
ST = Stanza
L = Line
ST 1, L4 - 'either' isn't needed here because you have just started a new sentence and are not comparing the crack to anything else yet.
ST 5 L1 - you need a comma after 'cathedrals' otherwise you are implying that the cathedrals are tinted and not the glass.
ST 6 L1 - again a comma is needed here after 'lenses' for the same reason as above.
I love the contrast between the different elements of the glass with the granite, clay and chalk. You have done well to continue your metaphor throughout the piece.
Personally I would remove the third lines of both ST 5 and 6, as they seem unneeded in a way, and almost get in the way of the message you are trying to bring through.
Overall, a really good piece.