This specimen is an example of a rock that formed in a magmatic environment. 

Magmatic: Gabbroic Minerals
Magma is a complex solution composed chiefly of silicate minerals and dissolved gases such as water vapor and carbon dioxide.

They type of magma rich in magnesium and iron is called a mafic magma. As the magma slowly cools, crystallization takes place; which minerals develop depends on the temperature reached. At 1200°C, the first stage of crystallization, minerals such as olivine, feldspar, pyroxene, and spinel typically develop and make up a common kind of igneous rock called gabbro.

Mafic magmas, formed in Earth’s upper mantle, sometimes contain significant amounts of periodot, garnet (grossular), and diamond known to form at great depths. Other minerals such as magnetite, chromite, pyrrhotite, and platinum-bearing minerals can develop in commercially important amounts as the mafic magma moves upward and crystallizes in Earth’s cooler crust.

Gabbros and other similar igneous rock types, and the minerals they contain, are typically dark in color.