Examples of naturally formed solid structures include glass, salt, ice and coral reefs. These are produced by minerals subjected to heat or compression, water exposed to extremely low temperatures, and as a residual feature of tiny ocean animals, respectively.
Naturally formed glass, called obsidian, comes from volcanoes. It is usually found along the edges of a volcanic flow or rimming a crater. Obsidian is also formed when lava contacts water or cools rapidly while airborne. Black is the most common color, but obsidian also comes in brown, tan, green or in multishades.
Halite, or salt, is formed by water evaporation and then compression. Most of this takes place along the ocean but exceptions include the Great Salt Lake in Utah and the Dead Sea shared between Jordan and Israel. Some deposits are thought to be thousands of feet thick.
Naturally occurring ice is made of frozen ocean water and compacted snow. The salt in ocean water doesn't freeze and is expelled, leaving crystals of mostly fresh water behind. The crystals gather on the surface and create ice sheets. Glacier ice is made of compacted snow. It becomes sea ice when icebergs calve off the glaciers and begin floating away.
The Great Barrier Reef is an example of a natural solid structure with organic beginnings. Coral polyps build individual "homes." Over time these homes create underwater reefs that support vast numbers of marine creatures. The Great Barrier Reef is so large that it can be seen from space.