Rock&Gem Magazine|September 2021
Benitoite’s Uncommon Partner

Named for the Roman Sea God, Neptunus, neptunite is often found with California State’s official gem mineral benitoite. Neptunite is found with such a bright blue gemmy associate that it is so popular and rare that everyone wants to own benitoite. No wonder neptunite has not been given the attention it deserves. It is understandable that benitoite tends to eclipse neptunite for attention. But neptunite is a beautiful mineral in its own right. It appears as a lustrous, black, opaque crystal on a stark white matrix. Crystals can measure up to two inches with perfectly terminated prismatic faces in nice-sized display specimens.

Neptunite is found in the San Benito Mountains, locked in a contrasting snow-white natrolite matrix. The specimens are very attractive and well worth collecting as the lustrous stark black crystals contrast nicely with blue benitoite and white natrolite matrix.

Neptunite is very complex chemically composed of sodium, potassium, lithium, iron, manganese, titanium silicate. It is also found elsewhere as a high manganese neptunite and so-called mangan-neptunite. This forms when manganese substitutes for some of the iron atoms in neptunite’s chemistry.

The neptunite from California is the iron end member of a series, with mangan-neptunite the opposite member. Localities that yield mangan-neptunite are nepheline syenite pegmatite deposits, unlike the San Benito source.

Benitoite is a barium titanium silicate and it isn’t as chemically complex as neptunite. They form together in a hydrothermal replacement deposit made up of glaucophane schist and serpentine and the typical rock types in the San Benito Mountains, which are part of the Diablo Range. The entire range is composed mainly of metamorphic-type rocks that can also contain chrysotile asbestos. The composition of these mountains has been an important factor in recent years as we became more aware of the danger of fibrous minerals like asbestos. This has caused the government to close the entire Diablo Range for a two-year study to determine if the rocks are hazardous. Even the roads were closed, restricting travel and reducing dust. The area has since been opened with some restrictions.

The benitoite-neptunite deposit, usually called the Dallas Gem mine is also subjected to health controls now, and what used to be a place where rockhounds could enjoy a weekend of digging and claim owners could use heavy equipment was suddenly subjected to serious controls. No more heavy equipment is allowed. No blasting is possible and gem mining is really restricted to hand tools due to environmental concerns. The deposit today is still claimed and some collecting is possible.

This unique deposit of benitoite and neptunite is a case of an accidental discovery. A prospector looking for one valuable mineral accidentally discovered another. Unlike most prospectors in California over 100 years ago, this prospector was not looking for gold but mercury! But even his search had its beginnings with the discovery of California gold in 1848.

When native gold is found in streams and rivers, the initial finds are tiny nuggets and flakes large enough to see so they can be separated from the sands and gravels with ease. But for centuries, prospectors have known they were missing the tiniest, nearly invisible flakes of gold, so when possible, they employed liquid mercury to capture the unseen gold or as known in prehistoric times, the oily fleece of a sheep, hence the “golden fleece” legend.

Continue reading your story on the app

Continue reading your story in the magazine