Three metals are listed – gold, silver and brass. Gold, the most precious, was used to coat the Ark. The inside was wood.
Why were both inside and outside not made of gold? Surely gold is not only more valuable but more durable.
A good argument could be advanced for making the interior of gold too, especially since the Torah, placed in the Ark, is more precious than gold (Psalm 19:11). Wisdom is too (Prov. 16:16). Did God have some special reason for the use of wood inside the Ark?
It may be that wood, like gold, has a special quality with a special relevance to the repository of the Divine Word.
Gold symbolises preciousness and permanence, but it is inanimate. It does not breathe or develop as wood does. The Word of God is etz chayyim, a tree of life (Prov. 3:18). It needs to be protected from menaces from outside, hence the gold. But in itself it lives and gives life.
If the whole Ark were gold we might turn it into an icon and worship the container and forget about the contents. We must make sure we look at both the flask and at what it contains.