[Atomic-S]

Does anyone know what kind of mineral these crystals are, and on what kind of substrate?

[CliffordK]

Quartz forms hexagonal clear and whitish crystals. 

I'm guessing the brown is some kind of sedimentary aggregate. 

[RD]

the pale hexagonal bits look like calcite ...

http://www.nms.ac.uk/collections/details.php?item_id=261747&terms=

http://www.geodil.com/image.asp?ImageID=672&history=0&categoryid=27


Link to inset image (bottom right corner) ... http://www.huntsearch.gla.ac.uk/cgi-bin/foxweb/huntsearch/DetailedResults.fwx?collection=all&SearchTerm=M1717&mdaCode=GLAHM&reqMethod=Link


Calcite will dissolve (and bubble) in acid, Quartz wouldn't.

https://en.wikipedia.org/wiki/Calcium_carbonate

[Sprool]

calcite tends to be a lot more milky in appearance, quartz is normally more translucent. Calcite is much softer (Moh 3 compared to quartz 7)

[Bass]

Atomic-S, can you give us any more information?  Not calcite, could be quartz, or could be a number of other minerals (sillaminite, for example).  Substrate, at least in that photo, appears volcanic, metamorphic or greisen.

[JimBob]

Someone may be trying to pull a fast one. I tried to download the pict to my computer to enlarge it and -- GUESS WHAT ???

It is posted on the web - it is a web link from Hunterian Museum & Art Gallery Collections: Item GLAHM M1717, Glasgow.

The ID on the we page is "calcite; cream crystal, showing prism and rhombohedron forms, on fluorite, with minor pyrite ."

See the link below the picture. The enlarged picture shows therhohmbid structure formed by one square crystal penetrating another.

[RD]

Someone may be trying to pull a fast one. I tried to download the pict to my computer to enlarge it and -- GUESS WHAT ???

It is posted on the web - it is a web link from Hunterian Museum & Art Gallery Collections: Item GLAHM M1717, Glasgow.

No skullduggery, that was me: I stuck a copy of The Hunterian Museum calcite specimen in the bottom right corner of a copy of the original poster's image for comparison, (it seemed a very good match). 

]

[JimBob]

This is from http://www.minerals.net/mineral/calcite.aspx

I didn't make this up, folks. Calcite is not an isometric mineral, rather it is a hexagonal mineral  with (normally) rhombohedral cleavage.  (Bass - YOU are a better mineralogist than I ever thought to be. Fur shame, fur shame !.)
Chemical Formula    CaCO3
Composition    Calcium carbonate, sometimes with impurities of iron, magnesium, or manganese, and occasionally zinc and cobalt.
Variable Formula    (Ca,Fe,Mg,Mn,Zn,Co)CO3
Color    Colorless, white, yellow, brown, orange, pink, red, purple, blue, green, gray, black. May also be multicolored or banded.
Streak    White
Hardness    3
Crystal System    Hexagonal
Crystal Forms
and Aggregates    Occurs in a great variety of shapes, with the most common forms asrhombohedral and scalenohedral crystals. Crystals may be tabular, acicular, prismatic, flaky, and needle-like. May occur as bundles of scalenohedrons, intergrown rhombohedrons, hair-like masses of acicular crystals, grainy, stalactitic, fibrous, massive, and earthy. Scalenohedral twinning is common.
Transparency    Transparent to opaque
Specific Gravity    2.7
Luster    Vitreous
Cleavage    1,3 - rhombohedral
Fracture    Conchoidal. Rarely observed due to the perfect cleavage.
Tenacity    Brittle
Other ID Marks    1) Commonly fluorescent; specimens from different localities fluoresce different colors. Some Calcite is also phosphorescent.
2) Transparent crystals exhibit strong double refraction.
3) May be thermoluminescent.
Complex Tests    1) Effervescent in hydrochloric acid and most other acids.
2) Calcite that doesn't fluoresce usually becomes fluorescent upon heating.
In Group    Carbonates; Calcite Group
Striking Features    Hardness, cleavage, fluorescence, and effervescence with hydrochloric acid.
Environment    Calcite is a constituent of all mineral environment, including sedimentary, igneous, and metamorphic.

[Atomic-S]

RD: I agree that your picture has a striking resemblance to mine, and I suspect you have correctly identified it.