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 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 asphalterupted 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.
Brick, too, is an artificial rock—in this case, an artificial form of massive slate. (See the Artificial Rocks Gallery for more examples.)
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.
Did you know that the diamond is the hardest natural substance found on earth? Did you know that quartz is one of the most common minerals on Earth? Did you know that marble forms from metamorphosed carbonate rock, most usually limestone? Did you know that basalt is the most common rock on Earth? Did you know that breccia is a rock composed of generally large, sharp fragments cemented together? Did you know feldspars make up more than 50% of the Earth's crust? Did you know the first recorded use of turqouise dates back to 5000 BC in Mesopotamia, where people used the gemstone to make beads? Did you know lapis lazul is treasured for its rich blue color and is often used in jewelry? Did you also know that ancient Eqyptians used powdered lapis lazul as eye shadow? Did you know jade because of it's toughness has been used for many cultural things like hammers, fish hooks, and stone axes. Did you know that rubies are one of the most popular gemstones today. Did you know that the first geologist on the moon was Harrison Schmitt who was part of the Apollo 17 mission? From the rock samples he collected, scientists have been able to learn many things about the moon. Did you know that the Earth is approximately 4.8 billion years old? Did you know that gold is so soft and easily worked that you could roll an ounce of it into a hair-thin wire 50 miles long? Did you know that the biggest pure-gold nugget was found in Australia in 1869 and weighed 156 pounds? Did you know that platinum is so rare that two million pounds of ore may contain only one pound of metal? Did you know that geodes are dull balls of igneous or sedimentary rock on the outside, but contain beautiful crystals on the inside? Did you know that meteorites, rocks from space, help scientists learn about the solar system and are very valuable, too? Did you know that the Taj Mahal built between 1632 and 1654 in India is made entirely out of marble? 
Andesite—The typical intermediate arc lava.
Basalt—A bubbly, crystally chunk of this igneous rock from Kilauea.
Diorite—A plutonic igneous rock between granite and gabbro.
Felsite—The volcanic version of granite.
Gabbro—The crystallized, plutonic version of basalt.
Granite—The rock that makes up the bulk of the continents.
Granodiorite—Almost diorite, except that this rock has quartz.
Gypsum Rock—A crusty chunk from a Nevada roadside.
Lapillistone—A volcanic rock formed of little ash balls.
Obsidian—Hand specimen of this useful volcanic glass.
Pahoehoe—Smooth-skinned flows of basalt lava.
Pegmatite—The igneous rocks with the biggest crystals.
Peridotite—Dark, dense, rarely seen rock from the Earth's mantle.
Perlite—A lightweight volcanic rock of great usefulness.
Porphyry—An igneous rock with large mineral grains in a fine matrix.
Pumice—A rock made from volcanic eruption products.
Pyroxenite—An uncommon ultramafic rock from the deep sea crust.
Quartz Monzonite—Just like granite, except it's short on quartz.
Rhyolite—The lava type with the highest silica content.
Scoria—A frothy volcanic product much like pumice.
Syenite—A plutonic, alkalic "granite-without-quartz."
Tuff—A rock made from volcanic ash.
Alabaster—A polished slab of this beautiful stone.
Arkose—A raw sandstone made from eroded granite.
Asphalt—Natural, from a natural oil seep.
Banded Iron Formation—Dazzling, extremely ancient "tiger iron."
Breccia—Rock made from broken rocks, breccia has many forms.
Chert—A common siliceous rock type with much variety.
Coal—The original fossil fuel.
Conglomerate—Rocks made with extra-big grains.
Coquina—A limestone made from shell fragments.
Dolomite—An altered near-twin of limestone.
Graywacke—An impure sandstone also called wacke.
Limestone—The greatest carbon-bearing rock.
Peat—Brown, vegetative precursor to coal.
Sandstone—Where sand goes to and comes from.
Shale—Rock made from mud and clay.
Siltstone—Made from sediment finer than sand.
Travertine—A form of limestone created in springs.
Blueschist—Rock made by high-grade metamorphism in subduction zones.
Eclogite—The most extreme metamorphic rock you can find.
Gneiss—Banded, tough, it forms the lower crust.
Greenschist—Rock made by low-grade metamorphism of various rock types.
Greenstone—A dark green rock made by metamorphism of basalt.
Marble—The metamorphic version of limestone.
Migmatite—A swirly rock produced by extreme metamorphism.
Phyllite—Shiny, colorful rock made by metamorphism of slate.
Quartzite—Rugged rock made by metamorphism of sandstone.
Schist—Striped rock made by metamorphism of mudstones.
Serpentinite—Green, scaly metamorphosed ocean crust.
Slate—Platy rock made by early metamorphism of shale.
Fulgurite—Lightning striking sand makes this curious object.
Geodes—These rock-shop lumps have hollow, glittering interiors.
Iron Meteorite—Pieces of the cores of ancient asteroids.
Nakhlite (Mars Meteorite)—A Martian meteorite that fell of 1911.
Shergottite (Mars Meteorite)—A meteorite from the Martian mantle.
Tektite—A meteorite made from Earth's crust by large impacts.
Thunder Egg—Solid, agate-filled lumps found in certain volcanic lands.
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