The flammability of magnesium is well-known. Magnesium ignites around 500 degrees Celsius, burning with a temperature of 3100 degrees Celsius. It is highly reactive, burning in oxygen, nitrogen, and carbon dioxide. It also reacts with water and can burn in small amounts of it. (Large amounts of water still extinguish small amounts of magnesium.)
Curls of magnesium can be ignited by a 3 inch magnifying glass on a piece of wood. They can also be ignited by sparks from burning ferrocerium, the material used to ignite cigarette lighters. This is the principle behind the camp firestarter.
Magnesium burns vigorously when mixed with most metal oxides. Even sodium hydroxide burns with magnesium, forming impure sodium metal. Metal oxides of less reactive metals such as copper forms violent mixtures with magnesium powder. These mixtures burn best when finely powdered and ground together.
Magnesium is extinguished quite rapidly in an enclosed location, but not before it burns some nitrogen, forming magnesium nitride. This produces an ammoniacal smell when reacted with water.
Burning magnesium powder explodes when struck by a fine stream of water. This can be dangerous, especially if the original intent was extinguishing the fire.
When a magnesium strip is placed in water and ignited at the top, it burns down until it hits the water, where it continues burning for a while, forming hydrogen gas from the water. Eventually, the water pulls enough heat from the magnesium to stop the combustion.
Magnesium does not readily form sparks when ground in air, but I did get a few sparks. They could have been from impurities (e.g. iron) on the grinding wheel.