From The Gemology Project
Revision as of 12:05, 31 October 2007 by Doos (talk | contribs) (Diagnostics: added note on carbonates being highly birefringent)
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Chemical composition FeCO3
Crystal system Trigonal
Habit Rhombohedrons, ganular, oolites
Cleavage Perfect, rhombohedral {101}
Hardness 3.5 - 4.5
Optic nature Uniaxial -
Refractive index 1.633 - 1.875
Birefringence 0.240
Specific gravity 3.83-3.88 (upto 3.96)
Lustre Vitreous, pearly, silky or dull
A rare faceted siderite. Notice the, easily visible, doubling of facet edges.
Photo by Chaman Golecha, Gem Testing Laboratory, Jaipur


Trigonal system.
Habit: rhomb shaped; massive; granular; globular; oolitic

Chemical composition

FeCO3 - iron carbonate.
Siderite belongs to the calcite group and forms series with rhodochrosite and magnesite.


This stone is too soft to be cut and is hence rarely faceted as a gemstone. As it is a carbonate, it would give effervescence (bubbling) when in contact with acids; strong eye visible doubling due to the high birefringence. Siderite, like most carbonates, is highly birefringent.


Pistachio brown (gem). Also pale yellowsih brown; pale yellowish; pale green; greenish gray; grayish brown; reddish brown; blackish brown; rarely almost colorless. Also massive.


Transparent to opaque.

Specific gravity

S.G.: 3.83-3.88 (upto 3.96), siderite will sink in all common heavy liquids.


Optic nature: birefringent, uniaxial negative.
Refractive index: nω = 1.875, nε = 1.633.
Maximum birefringence: 0.240.


Not very characteristic; band at 530, 500nm; vague at 440-480; strong 450, 475, 420 giving a cut-off like appearance.


UV fluorescence: generally inert.


Strong doubling of inclusions and pavillion facet edges due to very high birefringence.


Geological occurrence:
A widespread mineral in sedimentary deposits; hydrothermal ore veins; also in pegmatites; basaltic rocks.

Geographical locations:
Pinasquevia, Portugal: brown fine transparent material; Also Colarado; Conectiut; Idaho; Austria; France; Germany; Italy; Brazil (large fine); Quebec, Canada (brown rhombs); Greenland (rich brown, gemmy in cryolite); England, Cornwall (transparent known as 'Chalybite').