This small carving is made of ancient fossilized brain coral (Symphillia, formed in the Miocene era 6-10 million years ago), and comes from the Atoni culture (also known as the Atoin Meto, Atoin Pah Meto, and Dawan) of Indonesian West Timor. As I understand it, masks like this were traditionally thought to contain the spirit of ancestors, and were kept inside the house to provide protective spirits for the home. (Relatedly, there is some really interesting writing out there about the spiritual beliefs and rituals informing the very specific design of Antoni homes.)
There are much larger and more sculptural Atoni coral marks out there, but I rather like the simplicity of this little one, with an expression that seems both surprised and keenly alert. It measures 3" wide x 3 1/16" tall x 1 3/4" thick and in very good condition.