The blue, violet, purple end of the visual spectrum is not one where rocks and minerals shine. In the field, you are unlikely to see minerals in this color range unless you are in one of four settings: in order of abundance they are pegmatites, certain metamorphic rocks, oxidized zones of ore bodies, and low-silica (feldspathoid bearing) igneous rocks. This list is for minerals that are typically or most characteristically blue, violet, purple or related shades. I've looked at a lot of rocks, and I've seen only half of these minerals in the field. Nevertheless they are easily collected in good specimens at rock shops.
Inspect a bluish mineral carefully in good light. Decide the best name for its colorthe colors of the minerals in this list include blue-green, sky-blue, lilac, indigo, violet and purple. In translucent minerals, blue color is less reliable than in opaque minerals. At the same time, note the mineral's hardness and its luster on a fresh surface. If possible, determine the rock classigneous, sedimentary or metamorphicand the more specific rock type as well as you can (see "How to Look at a Rock" for starting guidance).
Geology Guide photo
Apatite is an accessory mineral found as crystals in many pegmatites. It is often blue-green to violet, although it has a wide color range from clear to brown befitting its wide range in chemical composition. Luster glassy; hardness 5.
An accessory mineral of high-magnesium, high-grade metamorphic rocks, cordierite forms grains that display a color that shifts between blue and gray as you turn it. This unusual feature is called dichroism. If that isn't enough to identify it, cordierite is commonly associated with mica minerals
, its alteration products. Luster glassy; hardness 7 to 7.5.
This uncommon boron silicate occurs as fibrous masses in pegmatites, in gneisses and schists, and as needles embedded in knots of quartz
in metamorphic rocks. Its color ranges from light blue to violet. Luster glassy to pearly; hardness 7.
This amphibole mineral most often is what makes blueschists
blue, although bluish lawsonite and kyanite may also occur with it. It is widespread in metamorphosed basalts, usually in felted masses of tiny needlelike crystals. Its color ranges from pale gray-blue to a "blue-jean" indigo. Luster pearly to silky; hardness 6 to 6.5.
Aluminum silicate forms three different minerals in metamorphic rocks (pelitic schist and gneiss
), depending on the temperature and pressure conditions. Kyanite, the one favored by higher pressure and lower temperature, typically has a mottled light blue color. Besides the color, kyanite is distinguished by its bladed crystals with a unique property of being much harder to scratch across the crystal than along its length. Luster glassy to pearly; hardness 5 lengthwise, 7 crosswise.
Lepidolite is a lithium-bearing mica mineral found in select pegmatites. Rock-shop specimens are invariably lilac colored, but it may also be grayish green or pale yellow. Unlike white mica
or black mica
, it makes aggregates of small flakes rather than well-formed crystalline masses. Look for it wherever lithium minerals occur such as colored tourmaline or spodumene. Luster pearly; hardness 2.5.
Oxidized Zone Minerals
Deeply weathered zones, especially those at the top of metal-rich rocks and ore bodies, produce many different oxides and hydrated minerals with strong colors. The most common blue/bluish minerals of this type include azurite
, chalcanthite, chrysocolla
, linarite, opal
and vivianite. Most people will not find these in their own neighborhood, but any decent rock shop will have them all. Lusters earthy to pearly; hardnesses 3 to 6.
Purple or violet quartznamed amethyst as a gemstoneis found crystallized as crusts in hydrothermal veins
and as secondary (amygdaloidal
) minerals in some volcanic rocks. Amethyst is quite uncommon in nature, and its natural color may be pale or muddled. Iron impurities are the source of its color, which is heightened by exposure to radiation. Luster glassy; hardness 7.
Alkaline low-silica igneous rocks may have large masses of sodalite, a feldspathoid mineral that usually has a rich blue color, also ranging from clear to violet. It may be accompanied by the related blue feldspathoids hauyne, nosean and lazurite
. Luster glassy; hardness 5.5 to 6.
A lithium-bearing mineral of the pyroxene group
, spodumene is restricted to pegmatites. It's typically translucent and commonly takes on a delicate lavender or violet shade. Clear spodumene can also be a lilac color, in which case it is known as the gemstone kunzite. Its pyroxene cleavage
is combined with a splintery fracture. Luster glassy; hardness 6.5 to 7.
Other Blue Minerals
Benitoite Geology Guide
There are a handful of other blue/bluish minerals that occur in various uncommon settings: anatase (pegmatites and hydrothermal), benitoite
(one occurrence worldwide), bornite
(bright blue tarnish on a metallic mineral), celestine
(in limestones), lazulite (hydrothermal), and the tanzanite variety of zoisite (in jewelry).
A large number of minerals that are usually clear or white or other colors may be occasionally found in shades from the blue to violet end of the spectrum. Notable among these are barite
, blue quartz
, brucite, calcite