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Rock and Mineral Dictionary |
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dacite Dacite is a type of volcanic rock that is light-colored and rich in silica (63 to 68 percent). |
danburite Danburite (Calcium borosilicate - CaB2Si2O8) is a clear to white silicate mineral whose orthorhombic crystals are transparent to translucent (danburite can also be yellow, greenish, or brown); it resemblestopaz. It was named for the city of Danbury in Fairfield County, Connecticut, USA (where the original specimens were found in 1839). Danburite is also found in Russell, New York (USA), Charcas and San Luis Potosi (Mexico), Kyushu Island (Japan), Madagascar, Siberia, Mogok (Myanmar), Bolivia, and Uri (Switzerland). Danburite has a hardness of 7 - 7.3 and a specific gravity of 2.97 - 3.02. Its streak is white. |
Darya-i-Nur Darya-i-Nur (meaning "Sea of Light") is one of the largest-known diamonds. It is a flawless, transparent, pink diamond from India, weighing about 175 to 195 carats. It was taken to Prrsia (now Iran) after Persia's attack on Delhi, India, in l739. The Darya-i-Nur is in the crown jewels of Iran, and was worn by the Shah of Iran. |
debris avalanche A debris avalanche is a sudden rock/soil/debris slide and flows with great speed from a volcano. |
debris flow A debris flow (also called a mudflow or lahar) is a moving mixture of rock, water, and other debris. |
decibel Decibels (abbreviated dB) are the unit of measuring the loudness (or intensity) of sounds. 0 dB is the faintest audible sound. The decibel scale is logarithmic. Blue whale calls are up to 188 dB; this is the loudest animal sound, and is painful to the human ear. Volcano eruptions are one of the loudest sounds on Earth and are over 272 dB. |
degree A degree is a measure of temperature or angle. |
demantoid garnet Demantoid garnets are valuable green, very lustrous garnets with a cubic crystalline structure. They are a rare variety of andradite. Demantoid garnets have characteristic inclusions that look like horsetails. Demantoid garnets have a hardness of 6-7 and a specific gravity of 3.8 - 3.9. Demantoids were very popular in the 1800's, but are rarely used today. |
dendritic Dendritic means tree-like, having a branching pattern (like moss agate). |
denim lapis Denim lapis is a relatively pale, inexpensive variety of lapis lazuli that is from Chile. It is the color of denim cloth due to calcite inclusions (which whiten the stone and lower its value). |
density Density is the weight per unit volume of a material. |
diamond Diamonds are precious, lustrous gemstones made of highly-compressed carbon. Diamonds are one of the hardest materials known. Diamonds have a hardness of 10, a specific gravity of 3.5, and a refractive index of 2.417 - 2.419. Colors of diamonds range from colorless, yellow, orange, brown, to almost black. Rarer colors are red, blue, green, and purple; these colors (called fancies) are quite valuable. Canary diamonds have a deep yellow color. A diamond's value is based on the "4 C's": color, cut, clarity, and carat weight. A diamond's color (saturation) is rated on an alphabetical scale ranging from D (white) to Y (yellow). "Z" diamonds are fancy, or deep-colored diamond. A diamond's cut is designed to maximize the stone's natural "fire"; brilliant cuts are preferred. A diamond's clarity depends on the number and size of its flaws and inclusions (of other minerals, like quartz). Clarity is rated from FI (flawless), IF (flawless at 10x magnification), a series of V ratings (very small flaws at 10x magnification), a series of S ratings (small flaws at 10x magnification), to I1, I2, and I3 (having inclusions visible to the naked eye). A diamond's carat weight is simple how much it weighs (a carat is about 0.2 grams or about 0.007 ounces). The largest-known gem-quality diamonds include the Cullinan (aka the Star of Africa, 530.20 carats), the Excelsior, the Great Mogul (an ancient Indian diamond which is said to have originally weighed 787.5 carats, but its location is not not known and nothing about it has been authenticaed), the Darya-i-Nur, the Koh-i-Nur, and the Hope diamond (named for a purchaser, Henry Thomas Hope). |
diatreme A diatreme is a volcanic vent or pipe that is formed by gas-charged magma. |
dichroism Dichroism is the property of having more than one color, especially when viewed from different angles. Many minerals (like rubies and axinite) are naturally dichroic. This effect can be artificially caused by a thin layer of a metallic oxides that is deposited on the surface of a material. Dichroic coated glass transmits some wavelengths of light and reflecting others, giving it an opal-like appearance. |
diffusion treated Diffusion treated stones are color-enhanced (not naturally colored) stones. The diffusion process only colors the outer surface of the stone, so chipping or repolishing will result in a loss of color. Diffusion-treated stones are already-cut stones that are heated in the presence of other compounds (like iron oxide, chromium oxide, titanium dioxide, etc.) that will infuse the extreme outer surface stone with color. Under a microscope, you you can see the loss of color within each tiny scratch. Diffusion treatment can also change the stone's refractive index. Also, if the stone is faceted, the color will appear stonger where the facets meet. |
dormant volcano A dormant volcano is a volcano that is not active now, but may erupt in the future. |
 doubly-refractive stone In doubly-refractive stones, lthe light entering the stone is split into two light rays, and the rays travel in different paths. These stones have more than one refractive index. Calcite, peridot, zircon, tourmaline, and titanite are doubly-refractive stones. Bi-refringence is another name for double refraction. |
druze Druze is a layer of crystals that form within a mineral crust, like the inner cavity of a geode. Amethyst crystals are often found in a druze. The inner cavity of agate geodes are often lined with a druze of sparkling quartz crystals. |
ductile A ductile substance is easily pulled or stretched into a thin wire. gold is the most ductile metal. |
dumortierite Dumortierite (Aluminum Boro-silicate Hydroxide) is a blue to violet silicate mineral that is used as an ornamental stone (and sometimes as a semi-precious stone in jewelry). Dumortierite quartz is a massive variety of opaque quartz that is intergrown with dumortierite crystals. Dumortierite has a hardness of 7 - 8.5 and a specific gravity of 3.3 - 3.4. |
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Rock and Mineral Dictionary |
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