Is the Shield of Expression made of wood or metal?

I have a Shield of Epression, and the description I have of it is the following:

The front of this shield is shaped in the likeness of a face. While bearing the shield, you can use a bonus action to alter the face’s expression.

If I check the description of a shield, I have the following:

A shield is made from wood or metal and is carried in one hand. Wielding a shield increases your Armor Class by 2. You can benefit from only one shield at a time.

What is the most logical material this specific shield could be made of?

I’m asking because in my last session, Heat Metal caused me a lot of troubles xD