Proustite

Augelite is a rare mineral that is semi-transparent and may take on several hues, ranging from blue to pink to green or colorless. Augelite crystals may be large enough to cut gemstones out of, but because the stone is both soft and brittle, it is rarely of interest to anyone beyond collectors.

Ferberite is a variety of Wolframite and is only separated from the rest of the family by its high iron concentrations. Surprisingly, its appearance lacks the classical coloration of other minerals with similar iron content. It is mined as an ore of tungsten, and collectible crystal specimens aren't common to come by.

Wardite is a rare and fairly obscure mineral that may be found in hues ranging from blue to green to milky to colorless. Because its crystals tend to take on a rare form (they are tetragonal trapezohedral), this mineral is often sought after by collectors. Soft and brittle, it is rarely cut into gemstones.

Powellite is an unusual, molybdenum-containing mineral that appears in a variety of colors, though it is most commonly whitish or yellowish. The mineral was discovered in Idaho in the 1890s and was named for John Wesley Powell, a famed geologist and explorer who spent much time delving into the mineralogy and geology of the American west. Powellite is not used industrially or commercially, though some exceptionally-colored specimens may be sought after by collectors.

Pectolite is a unique material known for its unusual crystals that form extremely slender fibers radiating from the central point outwards. This crystal habit makes the material very delicate and fragile — it can be easily broken and dented when touched. Pectolite is highly after by collectors, particularly the blue variety called larimar.

Titanite, also known as Sphene (especially when cut for gemstones), is a rare mineral occasionally mined as a source of titanium dioxide, used to create white pigments. Exceptional, gem-quality crystals, which are usually somewhere on the green-yellow spectrum, are highly sought after by collectors. Cut titanite has extremely high dispersion, or "fire" (i.e., white light passing through it is dispersed into many colors - diamonds are also famous for having high dispersion).

Tennantite is a usually dull, grey- or black-colored mineral that is a minor ore of copper, and evidence suggests that it has been smelted by humans to produce that useful metal since ancient times. First identified by geologists in Cornwall, England, tennantite is named after Smithson Tennant, an English chemist born in the 18th century.

Inesite is an appealing pinkish-brown translucent mineral that is sometimes confused with rhodonite. Its name comes from the Greek word for 'fiber' and refers to the sometimes fibrous appearance of this mineral. Inesite is uncommon to find in hydrothermal conditions often with manganese-rich metamorphic rocks. Its fracture is brittle and its streak is white.