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"Grape Agate", Grape-Like Chalcedony

 
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Macro_Cosmos



Joined: 15 Jan 2018
Posts: 163
Location: Sydney

PostPosted: Mon Feb 12, 2018 12:49 am    Post subject: "Grape Agate", Grape-Like Chalcedony Reply with quote

Grape Chalcedony by Macro Cosmos (DH)
Bigger version on Flickr.

Taken with the Laowa 25mm. I'd love to hear some advice on lighting, this is my current setup:



Diffusion provided by some plasticy-thingy bought at Ikea. DIY diffusion box made from those metal book holders.


Any advice is greatly appreciated! I find this to be harder than 1:2 and 1:1 settings, a lot harder with lower tolerances of failure. Very Happy
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soldevilla



Joined: 16 Dec 2010
Posts: 515
Location: Barcelona, more or less

PostPosted: Mon Feb 12, 2018 4:35 am    Post subject: Reply with quote

Are you sure that this is chalcedony? They look balls of fluorite.
Chalcedony is microcrystalline, never shows crystals, and quartz is not cubic.
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Macro_Cosmos



Joined: 15 Jan 2018
Posts: 163
Location: Sydney

PostPosted: Mon Feb 12, 2018 6:35 am    Post subject: Reply with quote

soldevilla wrote:
Are you sure that this is chalcedony? They look balls of fluorite.
Chalcedony is microcrystalline, never shows crystals, and quartz is not cubic.


It was called grape agate initially, which some people still call it that.
Most people on sites like mindat call it chalcedony now, I am not a mineralogist expert and I just go where the popular opinion goes.

https://www.mindat.org/min-51479.html

I do not think it is fluorite though. Fluorite has a very distinct look, these are actually individual clusters not attached to any kind of matrix.
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Pau
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Joined: 20 Jan 2010
Posts: 4550
Location: Valencia, Spain

PostPosted: Mon Feb 12, 2018 8:30 am    Post subject: Reply with quote

I agree with Soldevilla, quartz can't make cubic crystals, its internal structure doesn't allow it. Fluorite is of course a a possibility, or any other transparent mineral belonging to the cubic system.
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rjlittlefield
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Joined: 01 Aug 2006
Posts: 19411
Location: Richland, Washington State, USA

PostPosted: Mon Feb 12, 2018 12:01 pm    Post subject: Re: "Grape Agate", Grape-Like Chalcedony Reply with quote

Macro_Cosmos wrote:
I'd love to hear some advice on lighting, this is my current setup:

What you have looks pretty good, but I think you could do even better with a different diffusion material.

The reason is that I can discern your holding structure by looking in through the diffuser. That implies that your subject can discern your lights, which means that your diffuser really isn't diffusing very well, which leads to overly bright specular highlights.

I suggest substituting some thin paper in place of the plastic bubble sheets.

See http://www.photomacrography.net/forum/viewtopic.php?p=37343#37343 and the couple of posts after that for a quick comparison of plastic bubbles and paper for effectiveness of diffusion. Postings #1 and #3 of that thread, at http://www.photomacrography.net/forum/viewtopic.php?t=6090, show the effect of photographing the same subject with two different qualities of diffusion.

--Rik
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Lou Jost



Joined: 04 Sep 2015
Posts: 2898
Location: Ecuador

PostPosted: Mon Feb 12, 2018 3:55 pm    Post subject: Reply with quote

There looks to be a high ratio of glare to surface. For such subjects, besides better diffusion, I would recommend cross-polarization (with the lamp's polarizer placed on the subject side of the diffuser). And I would try a bit of illumination coming from behind and through the crystals.
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soldevilla



Joined: 16 Dec 2010
Posts: 515
Location: Barcelona, more or less

PostPosted: Mon Feb 12, 2018 4:25 pm    Post subject: Reply with quote

Macro_Cosmos wrote:

It was called grape agate initially, which some people still call it that.
Most people on sites like mindat call it chalcedony now, I am not a mineralogist expert and I just go where the popular opinion goes.

https://www.mindat.org/min-51479.html


I know the grape agate (I do not have a sample in my collection, but it will arrive soon, I hope)

I show a fluorite "ball" that really is a group of cubes. It was collected by me (yes, this piece is in my collection).

Fluorite has a very special diffraction index, which makes it difficult to illuminate. I think I used a disposable plastic cup of coffee, as a diffuser. If you try it, take a white balance because there are plastic cups that introduce a reddish dominant.

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JW



Joined: 23 Feb 2011
Posts: 145
Location: New Haven, CT, USA

PostPosted: Tue Feb 13, 2018 8:04 am    Post subject: Reply with quote

This material and its nature has been extensively discussed over at mindat.org

It is quartz, although chalcedony or some other variety may not be appropriate.

Its easy enough to test, fluorite is relatively soft and quartz is much harder.
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