Everyone knows what a rock is, until you ask what it is exactly. After some thought and discussion, most people will agree that rocks are more or less hard solids, of natural origin, made of minerals. But to geologists, all of those criteria have exceptions.
Rocks Are Hard
Not necessarily. Some common rocks can be scratched with your fingernail: shale, soapstone, gypsum rock, peat. Others may be soft in the ground, but they harden once they spend time in the air (and vice versa). And there is an imperceptible gradation between consolidated rocks and unconsolidated sediments. Indeed, geologists name and map many formations that don't consist of rock at all. This is why geologists refer to work with igneous and metamorphic rocks as "hard-rock geology," opposed to "sedimentary petrology."
Rocks Are Solid
Well, some are far from completely solid. Many rocks include water in their pore spaces. Many geodes—hollow objects found in limestone country—hold water inside them like coconuts. And the fine lava threads called Pele's hair, and the fine open meshwork of exploded lava called reticulite, are barely solids.
Then there's the matter of temperature. Mercury is a liquid metal at room temperature (and down to 40 below zero), and petroleum is a fluid unless it's asphalt erupted into cold ocean water. And good old ice meets all the criteria of rockhood too, in permafrost and in glaciers.
Rocks Are Natural
Not entirely. The longer humans stay on this planet, the more that concrete accumulates. Concrete is a mixture of sand and pebbles (aggregate) and a mineral glue (cement) of calcium silicate compounds. It is a synthetic conglomerate, and it acts just like the natural rock, turning up in riverbeds and on beaches. Some of it has entered the rock cycle to be discovered by future geologists.
Another human product that closely resembles rock is slag, the byproduct of metal smelting. Slag is a complex mixture of oxides that has many uses, such as in road building and concrete aggregate. It too has surely found its way into sedimentary rocks already.
Rocks Are Made of Minerals
Many are not. Minerals are inorganic compounds with chemical formulas and mineral names, like quartz or pyrite (see "What Is a Mineral?"). But what about coal? Coal is made of organic material, not minerals. The various types of stuff in coal are instead called macerals. Similarly, what about coquina, a rock made entirely of seashells? Shells are made of mineral matter, but they aren't minerals any more than teeth are.
Rocks like these are not controversial, but they have their own category: biogenic rocks. Perhaps concrete and slag could be added to that category too. Concrete would fit in with the others, being essentially sedimentary, but slag would probably be a biogenic igneous rock.
Finally we have the exception of obsidian. Obsidian is a rock glass, in which little or none of its material has gathered into crystals. It is an undifferentiated mass of geological material, rather like slag but not as colorful. While obsidian has no minerals in it per se, it is unquestionably a rock.