Type of rock

Basalt is a igneous rock that's hard, black, and made of fine-grained interlocking crystals. 

What does it look like?

Basalt is also made of minerals like olivine, pyroxene, and plagioclase, and contains less than 52% silica and is usually black or dark gray in color.

How/Where does it form?

Basalt forms when lava rich in magnesium and iron cools quickly near the surface of a planet or moon. 

Does it belong?

Basalt in this amount would NOT normally be found on an Island like Santorini (Thera).