Hi John
None that I know of, but note...
When I have used it to label bags or bottles (plastic or glass)
1) the surface needs to be clean & oil free.
2) If it comes in contact with alcohol such as: Ethanol, Isopropanol, Glycol,
Ethylene Glycol,  it tends to run, or become non-adherent and wipe off.

So this fact probably applies to shells as well.

I do find it great for labels!
I recomend using the Sanford Brand
Sharpie - Ultra Fine Point - Series 37000

Leslie

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Is there any good reason not to use Sharpie permanent marker pens on shells?
(It is a US brand of permanent marker pens which are much easier to use than
India ink)

Thanks for any info.
John Wolff
Lancaster, PA