The mishna Keilim 13:6 discusses the susceptibility to tumah of objects partially of wood and partially of metal.
עֵץ הַמְשַׁמֵּשׁ אֶת הַמַּתֶּכֶת, טָמֵא. וְהַמַּתֶּכֶת הַמְשַׁמֵּשׁ אֶת הָעֵץ, טְהוֹרָה. כֵּיצַד, פּוֹתַחַת שֶׁל עֵץ וְהַפִּין שֶׁלָּהּ שֶׁל מַתֶּכֶת, אֲפִלּוּ אַחַת, טְמֵאָה. פּוֹתַחַת שֶׁל מַתֶּכֶת וְהַפִּין שֶׁלָּהּ שֶׁל עֵץ, טְהוֹרָה. טַבַּעַת שֶׁל מַתֶּכֶת וְחוֹתָם שֶׁלָּהּ שֶׁל אַלְמוֹג, טְמֵאָה. טַבַּעַת שֶׁל אַלְמוֹג וְחוֹתָם שֶׁלָּהּ שֶׁל מַתֶּכֶת, טְהוֹרָה. הַשֵּׁן שֶׁבַּטַּס, שֶׁבַּפּוֹתַחַת, וְשֶׁבַּמַּפְתֵּחַ, טְמֵאָה בִּפְנֵי עַצְמָהּ:
Wood that serves a metal vessel is susceptible to impurity, but metal that serves a wooden vessel is clean. How so? If a lock is of wood and its clutches are of metal, even if only one of them is so, it is susceptible to impurity, but if the lock is of metal and its clutches are of wood, it is clean. If a ring was of metal and its seal of coral, it is susceptible to impurity, but if the ring was of coral and its seal of metal, it is clean. The tooth in the plate of a lock or in a key is susceptible to impurity by itself.
In the middle of the explanation, we see that coral (אלמוג) is treated as a type of wood. Is this a mistranslation? If coral is indeed treated as wood for tumah purposes, what else gets treated as wood that we would not normally think of as wood? That is, how broad is the class "wood"?