These pieces are simply studies of different materials. The first image is a series of materials meant to resemple real-world rocks and plants. From left to right I rendered granite, sandstone, basalt, obsidian, quartz, and grass. While somewhat stylized, the goal was to make each material recognizable.

The second piece is another stylized study meant to represent dirt/stone. Using a sphere provides a unique opportunity to easily experiment with lighting. As seen here, the ball is lit with a warm primary source and a cool secondary light.

The third image is an attempt at a more realistic rendering of the mineral galena. Studying minerals like this is a great way to observe somewhat simple geometric shapes in the real world.