Strengthen is very picky about what kind of stones it will work on.
Obsidian is okay, but opal, ruby and onyx aren't. Those are what I
can remember not working offhand.

Amber doesn't allow strengthn


<100%hp 92%m 100%mv 10495tnl (13.83%) (wilderness)> strength bow
You may not strengthen something of that material.

<100%hp 92%m 100%mv 10495tnl (13.83%) (wilderness)> exa bow
This bow is quite curious to behold. At first glance it seems to be made
of amber. It is a deep yellow and you can see fine bubbles hardened within
it. Strung with an acrid-smelling line of silk, the bow offers curious
resistance and seems rather difficult to break.

A yellow bow of hardened acid is a bow, made of amber, and weighs 9 pounds 15 ounces.

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