Our Gemara on Amud Beis tells us about the structure of the Holy Ark:

Bezalel made three nested arks: A middle one made of wood, whose height was nine handbreadths; an inner one made of gold, whose height was eight handbreadths; and an outer one of gold, whose height was ten handbreadths and a bit.

We might ask, how is this paying honor to the Holy Ark to have wood in the center.  Does it not seem a bit cheap?  Would you buy your wife a gold PLATED bracelet? Allow me a poetic expansion of an idea based on a Gur Aryeh (גור אריה על שמות כ״ה:י״א:א׳) :

The wood center represents the vital, growing, live part of Torah. External gold represents the way in which the hidden glory of the Torah is seen in this world. The internal gold is the true purity of the world to come. However, as with most things, the middle is where we all live.  Thus, the middle ark, the wood part, is the manifestation of Torah in this world. In our world, we study Torah and are constantly mechadesh, mining new ideas found in the Torah.  That is why wood is at the center.