Avoid marble in your lye solution as it will get eaten.

Sorry to quibble, but marble (and limestone) will not be eaten by lye;
lye
is alkaline, so is sodium carbonate in all its forms; it will,
however,
dissolve in an acid solution.

Eaten was a bit of an overstatement; nibbled at would be more correct.
Depending upon its source marble ranges from 30-60% calcium carbonate
and similar levels of magnesium carbonate which as you say will be
basically unaffected. However, marble can also contain up to 10% SiO2
which is etched and dissolved by hot concentrated alkali causing the
marble chips to slowly crumble, and up to a few percent Fe (and other
metals depending on the coloration) in various oxidation states which
will leach out and oxidize to rusty water - none of which I assumed that
the original poster would want in his homemade lye.

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Regards,
Carl Ijames carl.ijames at verizon.net