Bosnian term (edited): rožnac

Živković dr S, Građevinski rečnik:
rožnjak; rožnac - hornstone


Webster's Online Dictionary:

- Hornstone - A fine-grained metamorphic rock formed by the action of heat on clay rocks.
Building & Civil Engineering:
Cryptocrystalline impure silica. . . resembling flint. Black or grey when fresh and weathers to shades of brown or yellow. . . Occurs as beds and nodular concretions in lime stones. .
Synonym: hornfels (n).
(http://www.websters-online-dictionary.org/definition/hornsto...

- Hornfels - A fine-grained metamorphic rock formed by the action of heat on clay rocks.
Geological:
A dark, very fine-grained metamorphic rock produced by the recrystallization of a fine-grained rock by heat from a nearby igneous intrusion. From the German, meaning horn rock.
Mining:
A fine-grained rock composed of a mosaic of equidimensional grains without preferred orientation and typically formed by contact metamorphism. Porphyroblasts or relict phenocrysts may be present in the granoblastic matrix. See also:calc-silicate hornfels; pelitic hornfels;magnesian hornfels.
(http://www.websters-online-dictionary.org/definition/hornsto...

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Note added at 3 hrs (2006-05-13 23:23:10 GMT)
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A može i CHERT

Vukićević B, Građevinski rečnik:
rožnac - chert


Answers.com:
Hornstone (Min.) - A siliceous stone, a variety of quartz, closely resembling flint, but more brittle; -- called also chert.
(http://www.answers.com/topic/hornstone-1?hl=hornstone)


Webster's Online Dictionary:
Chert - Variety of silica containing microcrystalline quartz.
…
(http://www.websters-online-dictionary.org/definition/chert)