Although coarsely crystalline quartz occurs in colorless or white (milky) masses, colored varieties are numerous and popular. AMETHYST (violet); smoky quartz; cairngorm, or morion (black); citrine (yellow); and rose quartz are common and arise by the incorporation of a tiny fraction of elements that substitute for silicon atoms, such as iron, aluminum, manganese, and titanium. The entry of such elements requires the concomitant entry of small atoms such as hydrogen, lithium, or sodium to preserve charge balance. Inclusions of other minerals, in some instances oriented, can occur throughout quartz crystals. The included minerals can be RUTILE (sagenite or rutilated quartz); fibrous amphiboles (CAT'S-EYE is grayish green; TIGER'S-EYE is yellow brown; hawk's-eye is blue); and platy minerals, such as mica, iron oxides, or chlorite (aventurine). Very fine grained and cryptocrystalline varieties of quartz are numerous. Collectively called chalcedonic quartz, these varieties form slowly from evaporating or cooling solutions as crusts and fillings of veins and open spaces. When color banding is conspicuous, the variety is called AGATE. Agate with numerous flat bands of white, black, or dark brown is called ONYX. Translucent red or brown chalcedonic quartz colored by iron oxides; green varieties colored by chlorite, amphiboles, or nickel minerals; and mottled moss agates are used as semiprecious stones. BLOODSTONE is a green variety of chalcedonic quartz with red spots. Chalcedonic quartz is often colored by chemical processes. Finely crystallized quartz called CHERT AND FLINT occurs within calcareous or silty sedimentary rock as gray or black layers or nodules. JASPER is very fine-grained quartz with abundant incorporated iron oxides, it may be red, brown, yellow, dark gray, or black.