A pyroxene mineral found in igneous and metamorphic rocks. A minor gemstone.
What is Diopside?
Diopside is a pyroxene mineral with a chemical composition of MgCaSi2O6. It occurs in igneous and metamorphic rocks at many locations around the world.
Diopside has potential uses in the glass and ceramics industries but the mineral usually occurs in accumulations that are too small or impure for effective mining.
Gem-quality crystals of diopside are faceted into attractive gemstones that are occasionally seen in commercial jewelry. Granular diopside can be easily cut and polished. When it has an attractive color it is sometimes used as an ornamental stone.
Perhaps the most important use of diopside is its value as an indicator mineral in the search for diamonds. Trail-to-lode prospecting using diopside and other indicator minerals has found diamond deposits in Canada, the United States, Africa and other locations.
The most common occurrence of diopside at Earth's surface is as a primary mineral in olivine-rich basalts and andesites. In these rocks it can be present in quantities of a few weight percent.
Diopside is much more abundant in certain parts of Earth's mantle. Evidence for this is diopside as a common mineral in kimberlite and peridotite pipes formed during deep-source volcanic eruptions.
Diopside also forms during contact metamorphism of limestones and dolomites. Most of the crystalline diopside used to cut faceted gems and the granular diopside used as ornamental stone occurs in these carbonate deposits.
Physical Properties of Diopside
|Color||Grayish white, light blue to purple, light green to vivid green, brown, black.|
|Streak||White to light green.|
|Luster||Vitreous, sugary, earthy.|
|Diaphaneity||Opaque, translucent, transparent.|
|Cleavage||Two distinct directions, at 87 and 93 degrees; imperfect; prismatic.|
|Mohs Hardness||5.5 to 6.5|
|Specific Gravity||3.2 to 3.5|
|Diagnostic Properties||Cleavage, monoclinic crystal form.|
|Uses||Gemstone, diamond indicator mineral, potential industrial use in ceramics.|
Diopside as a Diamond Indicator Mineral
Most diamonds found at or near Earth's surface were delivered from the mantle during deep-source volcanic eruptions. These diamonds occur in vertical igneous structures known as pipes which are often composed of kimberlite or peridotite.
These pipes are difficult to locate. Their surface exposure is usually covered with soil and vegetation and it might be only a few acres in size. The pipes are often found by searching soils and sediments for mineral grains that are characteristic of the mantle rock that occurs in the pipe but are absent in typical surface materials. Small particles of chromium-rich diopside are bright green in color, are often abundant in the pipes, and are easy to recognize in surface materials. (It would be almost impossible to locate the pipes by looking for diamonds because they make up such a tiny fraction of the overall rock in the pipe and would then be diluted by local rock debris.)
Geologists use these green diopside fragments to locate the pipes. They know that the fragments are liberated as the pipe weathers, then scattered by the actions of mass wasting, streams and glaciers. When diopside fragments are discovered, the geologist knows that they originated up-slope, up-stream, or up-ice from the location in which they were found.
A trail of diopside fragments can lead the geologist to the pipe from which they were weathered. This activity, known as "trail-to-lode" prospecting, finds many diamond pipes and an even larger number of pipes without diamonds.
Diopside as an Industrial Mineral
Diopside has potential uses in ceramics, glass-making, biomaterials, nuclear waste immobilization and fuel cell technology. Unfortunately, natural diopside is rarely found in deposits that simultaneously have a size, purity, and location that allows economic mining. If diopside is used in these industries in significant volume, synthetic diopside might be competitive with diopside produced by mining.
Some crystals of diopside contain enough chromium to give them a rich green color. These can be cut into beautiful faceted stones, beads and cabochons. The appearance of these stones is best when they are under two carats because the material is often dark or strongly saturated.
Chrome diopside is not commonly seen in commercial jewelry. It has a rich green color that enables it to serve as an alternative gem for emerald at a significantly lower price. Diopside is rarely treated, unlike emerald which is often treated with various materials to seal and hide fractures.
One problem with chrome diopside is its durability. It has two directions of perfect cleavage and a Mohs hardness of only 5.5 to 6.5. This gives it a risk of being scratched or broken. The gem is best used in earrings, necklaces, brooches and other items that will not be subjected to abrasion or impact.
Even though chrome diopside is very attractive there are barriers to it becoming a popular gem that is widely seen in jewelry. First are durability concerns, second is that the jewelry-buying public is not familiar with the gem, and third is the fact that it is only available in limited supplies.
Some diopside crystals are filled with microscopic needle-shaped mineral grains that form a parallel alignment with the crystal structure of the mineral. These included mineral grains in alignment is known as a "silk". When this diopside is cut en cabochon the needles can reflect light much like how light is reflected from a spool of silk thread.
A silk with one direction of needle alignment will produce chatoyance, also known as a cat's eye. Silk with two or three directions of needle alignment will produce asterism. Two directions produces a four-ray star, and three directions produces a six-ray star. For these phenomena to appear, the stone must be cut with the needles oriented parallel to the bottom of the stone and the top of the cabochon must be symmetrically cut.
The mineral needles that form the silk are known in some instances to be magnetite. They are sometimes abundant enough to make the cut gems slightly magnetic. If you approach them slowly with a magnet the gems will move before the magnet touches them. The needles in some non-magnetic gems are thought to be rutile or illmenite.
Some diopside formed during the contact metamorphism of dolomite or limestone has a granular texture similar to marble. This material is known as "violane". It is often white, gray, light blue, lilac, or purple in color. Violane accepts a bright polish and is sometimes used to make cabochons, beads and ornamental items. Violane is a rare material in nature and almost never seen commerce.
Geographic Distribution of Diopside
Gem-quality chrome diopside and violane are mined in limited amounts in Siberia, Russia. Most of the chrome diopside used in jewelry today comes from a few locations in Siberia. Small occurrences of chrome diopside are also known in Austria, Brazil, Burma, Canada (Ontario and Quebec), Finland, India, Italy, Madagascar, Pakistan, South Africa, Sri Lanka, and the United States (New York), but none of them produce regularly or in significant quantities.
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