Upcoming Rockhounding Events in Colorado

Fri 10

Colorado Mineral and Fossil Fall Show

September 10 - September 18


Marble forms when limestone or dolomite is subjected to heat and pressure caused by thick layers of overlying sediment. This metamorphism can result in impurities in the limestone recrystallizing and forming mineral impurities that impact the texture and color of the marble. Desirable, pure-white marble is formed by very pure, silicate-poor limestone or dolomite.

Marble is an economically valuable rock. It is used in everything from fine arts to construction aggregate. Different locations produce marbles of varying quality or color that can be used for different purposes.

Marble is in the Public Domain

Smoky Quartz

Streak: White

Hardness: 7

Chemical Formula: SiO2

Smoky quartz is a brownish grey, translucent variety of quartz that ranges in clarity from almost complete transparency to an almost-opaque brownish-gray or black crystal. Like other quartz gems, it is a silicon dioxide crystal. The smoky color results from free silicon formed from the silicon dioxide by natural irradiation.


Streak: White

Hardness: 3.5 – 4

Chemical Formula: MnCO3

Rhodochrosite is a manganese carbonate mineral with chemical composition MnCO3. In its (rare) pure form, it is typically a rose-red color, but impure specimens can be shades of pink to pale brown. It streaks white, and its Mohs hardness varies between 3.5 and 4. Its specific gravity is between 3.5 and 3.7. It crystallizes in the trigonal system, and cleaves with rhombohedral carbonate cleavage in three directions. Crystal twinning often is present.

Rhodochrosite occurs as a hydrothermal vein mineral along with other manganese minerals in low temperature ore deposits as in the silver mines of Romania where it was first found. Banded rhodochrosite is mined in Capillitas, Argentina.

It was first described in 1813 in reference to a sample from Cavnic, Maramureş, present-day Romania.

Photo: Fluorite Quartz Rhodochrosite by Robert M. Lavinsky is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.

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Author: Celeste