Naquadah is a super heavy element with superconducting capabilities. It can store massive amounts of energy and is extremely hard; it was stated to be quartz like. It gives off a unique energy signature that the Goa'uld, former hosts, Jaffa, Tok'ra and Cassandra can detect. However large doses of drugs can counter this. Also handheld sensors, and those on the Prometheus can detect the signature. The energy stored in Naquadah can be released rather efficiently.
Small amounts of naquadah can be absorbed into the blood streams of some creatures. Allied with the host body's bio-electrical impulses, the energy contained within the naquadah in the bloodstream of the host and symbiote renders some Goa'uld devices operable when touched. Because of the naquadah in her blood Samantha Carter is resistant to anesthesia.
Radio signals cannot pierce large amounts of naquadah. Naquadah is capable of repeatedly handling an active wormhole, whereas titanium only lasts several minutes under similar conditions. The hardest known substance in the universe are the Asgard-created alloy, consisting of carbon, trinium, and naquadah.
Liquid naquadah is capable of firing powerful pulses of energy repeatedly. An isotopic variant of naquadah called naquadria stores much more power, but is radioactive. However it was believed to be created artificially by Thanos, and is only known to exist on Langara.