What Causes the Pink Color of Rose Quartz?
Rough Rose quartz.

Rose quartz is a type of quartz which exhibits a pale pink to rose red hue. The color is usually considered as due to trace amounts of titanium, iron, or manganese, in the material.

Rose quartz is commonly found in the quartz cores of pegmatites and is believed to form at high temperatures, but it has also been found in hydrothermal veins. They are found as a late formation in pegmatite pockets, often overgrowing smoky quartz crystals in groups of parallel-grown crystals.

star rose quartz.
Star rose quartz. Credit: Martin P. Steinbach

Rose quartz from some localities shows asterism when cut as spheres or cabochons, much like that seen in certain sapphires. It is sometimes called star rose quartz. The six-rayed star is caused by reflections of the light from embedded fibers that intersect at an angle of 60°.

The term "rose quartz" has also occasionally been used for quartz that is colored by other inclusions. Pale amethyst may be confused with euhedral rose quartz/pink quartz. Amethyst and euhedral rose quartz/pink quartz are both occasionally found overgrowing smoky quartz and may show similar sheet-like surface patterns.

Additionally, there is a rare type of pink quartz (also frequently called crystalline rose quartz) with color that is thought to be caused by trace amounts of phosphate or aluminium. The color in crystals is apparently photosensitive and subject to fading. The first crystals were found in a pegmatite found near Rumford, Maine, USA and in Minas Gerais, Brazil.

Rose Quartz is the South Dakota state mineral.

See also: 
What Is Faden Quartz, and How Does It Form?
What Causes the Purple Color of Amethyst?
Crystal Formations and Their Meanings