blueletterbible.org cites some authorities believing it to be cypress. The Vulgate merely states “lignis levigatis” — which is “smoothed wood.”
It looks like the Septuagint just says “xylon,” meaning “wood.”
“Gopher wood” does come from the Hebrew. It looks like the word is “gfr” (actually rfg but Hebrew is backwards), which can be translated as “pitch.” Translating it simply as “gopher” is misleading.
Now cypress does decompose very slowly.
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susie