Purple rocks, which may range in hue from blue to violet, get their color from the minerals those rocks contain. Although fairly rare, you can find purple, blue, or violet minerals in these four types of rocks, ordered from most to least common:
- Pegmatites composed primarily of large crystals, such as granite.
- Certain metamorphic rocks, such as marble.
- Oxidized zones of ore bodies, like copper.
- Low-silica (feldspathoid bearing) igneous rocks.
To properly identify your blue, violet, or purple mineral, you first need to inspect it in a good light. Decide the best name for its color or colors, such as blue-green, sky blue, lilac, indigo, violet, or purple. This is more difficult to do with translucent minerals than with opaque minerals. Next, note the mineral’s hardness and its luster on a freshly cut surface. Finally, determine the rock class (igneous, sedimentary, or metamorphic).
Take a closer look at the 12 most common purple, blue, and violet minerals on Earth.
Apatite is an accessory mineral, meaning it appears in small quantities within rock formations, usually as crystals in 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. Apatite is commonly found and is used for fertilizer and pigments. Gemstone-quality apatite is rare but it does exist.
Glassy luster; hardness of 5. Apatite is one of the standard minerals used in the Mohs scale of mineral hardness.
Another accessory mineral, cordierite is found in high-magnesium, high-grade metamorphic rocks like hornfels and gneiss. Cordierite forms grains that display a shifting blue-to-gray color 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 or chlorite, its alteration products. Cordierite has few industrial uses.
Glassy luster; hardness of 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. Dumortierite is sometimes used in the production of high-quality porcelain.
Glassy to pearly luster; hardness of 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 needle-like crystals. Its color ranges from pale gray-blue to indigo.
Pearly to silky luster; hardness of 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 hornfels than along its length. It is used in the production of electronics.
Glassy to pearly luster; hardness of 5 lengthwise and 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 in colored tourmaline or spodumene.
Pearly luster; hardness of 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, smithsonite, turquoise, and vivianite. Most people will not find these in the field, but any decent rock shop will have them all.
Earthy to pearly luster; hardnesses 3 to 6.
Purple or violet quartz, which is called amethyst as a gemstone, is found crystallized as crusts in hydrothermal veins and as secondary (amygdaloidal) minerals in some volcanic rocks. Amethyst is quite common 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. Quartz is frequently used in electronic circuitry.
Glassy luster; hardness of 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. It is primarily used as a gemstone or for architectural decoration.
Glassy luster; hardness of 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. Spodumene is the most common source of high-grade lithium.
Glassy luster; hardness of 6.5 to 7.
Other Blue Minerals
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, white, or other colors may be occasionally found in shades from the blue to violet end of the spectrum. Notable among these are barite, beryl, blue quartz, brucite, calcite, corundum, fluorite, jadeite, sillimanite, spinel, topaz, tourmaline, and zircon.
Edited by Brooks Mitchell