October 3-7—JOSHUA TREE, CALIFORNIA: Annual show; Sportsmans Club of Joshua Tree
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 Post subject: colour and that dreaded chapter 7! Help
PostPosted: Thu Nov 12, 2009 3:10 pm 
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Hi all,

I hope you are all well. I have been ploughing my way through the daytime dip course at Gem A london. All good so far. I haveeven managed to plough throuhg the colour section, and understand the details, the basic concepts and have put in about 25 hours of study this week. All dedicated to you name it: colour! I've got bloomin colour coming out of eyes and ears and i want to scream!!!
Band gaps!!!!! I get the details, but i don't get the concept or where it fits into place!!!Aaaaah - its driving me crazy! I get that it relates to conductiviy and that materials need a s**t load of energy for the e- to be able to condut electricity....I understand that colourless, 'pure', gems are quite happy in their valent state. Especially diamonds!!!
Then a diamond goes and has bloomin Nitrogen in it and everything changes!!! Its an impurity! I get that! So it absorbs energy in the VL range, at about 400nm!! i get that....but then it goes and jumps to the conductive band and i don't understand that!!!!
Is it that there is just a huge energy level difference? That N, needs anout 400nm of energy to jump to a higher state, which touches the conduction band???? I don't understand why this energy level is called 'A donor level', despite the fact that i understand that N has 5 e-, and hence one to many to bond with Carbon!!! I think i am going to explode...hehehe....
Then the same problem arises with Boron! i understand the mechanis of it, but don't understand why. Why, why, why???

Sorry to bog you down with my very confused brain...

Pixy x x x

T


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PostPosted: Thu Nov 12, 2009 3:19 pm 
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Pix, i did a chat on it last year in which I tried to explain the whole thing in understandable terms, see if it helps:

http://gemologyonline.com/Color_causes5.pdf

Read that first and then we'll see what needs further explanations.


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PostPosted: Thu Nov 12, 2009 4:48 pm 
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Thanks Tim...that was amasing! Cleared up a lot of gaps hehehe in my frazzled head!!! My god - you are better than the crap teachers at GemA! They should hire some help or get new teachers. I read a good thread from Doos too. He was awesome. So sad!
Anyway...- you are a clever gemmologist and fab and talented photographer. I feel like a novice now!! lol..... I was a violinist in a previous life, but gave it up to do this lol...
You been into gemmology long?

Thanks again....it really helped. i'll be reading it all over again moro, and before my exams...

Pix x


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PostPosted: Thu Nov 12, 2009 8:19 pm 
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Your diploma exam is in January?


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PostPosted: Fri Nov 13, 2009 5:20 am 
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yes it is in Jan 10! We haven't finished the syllabus yet! I'd say we've done 70% of it. This chapter was the hardest by far...:)
Pxx


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PostPosted: Fri Nov 13, 2009 5:54 am 
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Hi Tim...I have reread the discussion and have one question??

If the band gap is very small and is below visible light...would this make the material black?
When would the material be opaque?
Not sure where coloured opaque gems fits in??

Cheers

P x


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PostPosted: Sat Nov 14, 2009 6:46 am 
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Hi Pix,

Quote:
If the band gap is very smalland is below visible light...would this make the material black?


yups, in that case all the energy (visible light) that is fired at the material will do it's thing with the 'conduction electrons' (the ones that aren't tied up holding the substance together and can move freely through the material) and all light will consequently be absorbed. The result is an opaque (black) stone.

Quote:
When would the material be opaque?


There is many cases where sufficient light can be absorbed to make it appear opaque to our eyes. A small band gap is just one of 'm. A multitude of dispersed metal ions, colour centres or inclusions can have the same effect. Transparency is further related to the distance light has to travel. The bigger the rock, the less light comes out. It's a numbers thing there, the more electrons light will have to pass the more it will reduce in intensity. A material is opaque when the vast majority of the light interacts with the material. Multicrystallinity can be a cause as well. I'm sure I'm forgetting to mention a few other causes here too.

Quote:
Not sure where coloured opaque gems fits in??


You see color in an opaque material because of its reflected light, not transmitted light. Same same, but different. The material is opaque (black) when you put a tiny light behind it and try to see through the material but when viewed with the light behind you shining down onto the material its surface will reflect some of the light while using certain energy levels (light waves) to excite the electrons in the top layer one way or the other. The residual light you pick up will be registered as having a certain color.

have you read the entire chat series on color causes? You can find them on the left under 'index of previous chats'.

And thanks for the compliments, I haven't been involved in gemology that long, I did my diploma exam in june 2008. For the record, most of what I learned, and especially the things about color causes, have been coming from Gem-A... can't say it was crap... Blaiming the teacher is a bit easy don't you think? :wink:

That said, I wouldn't have been able to pass the exams without the help of Alain, Annie, Brian and the posters here on the forum. Free advice, knowledge and tutoring.... An amazing place this is...


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PostPosted: Sat Nov 14, 2009 10:45 am 
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Hi Tim,

Thanks again for your post....It helps to see things from different angles hehe...

i think i get a bit frustrated with the teaching at GemA because i have been a teacher for 15 years trying to improve and dedicate myself to understanding my students and how best to help them and feel a bit let down by the standard of teaching in GemA when it is such an expensive course. I also spend about 25 hours a week studying, every week and try to put every ounce of me into it. So its not out of lazyness either! By the sounds of it the teachers you got were better than mine lol.... I also got an A in my foundation so i must be doing something right...but i by no means think that means i will this time and know i have a lot of work to do to pass even.

Anyway - thanks for the help and fab explanations - you definitely have a knack for it and i am sure there are many on here that appreciate the enormous help you and others give, me included :D

pxxx


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PostPosted: Sat Nov 14, 2009 11:08 am 
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By the sounds of it the teachers you got were better than mine lol


I did it by correspondance. Apart from one practical lesson from Doug I never met any Gem-A instructors.

I found my teachers here when I needed them. :P


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PostPosted: Sat Nov 14, 2009 3:32 pm 
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Even better! You've excelled hehe.....

I did my foundation distance learning and was looking for a bit more input and better quality teaching on site in London! Its so expensive!! Despite the to's and fro's to London i am still glad i chose to do it on site though. I think on the practical front it really helps and meeting sharing gems and passion is quite fun too, and i get to do it in 5 months, which is great when you want to get going. I am itching to start.

I shall try and visit the site more often. Great site.

pxx


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PostPosted: Tue Nov 17, 2009 11:18 am 
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Hello everyone.
I will GEM-A exame in January.
Thanks to both for the explanations on this topic,it is very interesting for me too.
where i can read other topics like this , that can help me to to resolve any doubts (and pass the exam,ehehhe)
bye, Francesco


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PostPosted: Tue Nov 17, 2009 3:04 pm 
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Hi Alan...

welcome :D

I find this site really helps...

P xx


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PostPosted: Tue Nov 17, 2009 3:45 pm 
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Hi

Welcome :smt006

Here is where you will find other chats http://gemologyonline.com/chat_index.html

we also have chats on sunday that are co mixed with quantum physics , if you look to the left side of the screen you will see gemologyonline chat two links one for older chats and one for the room where we have our discussions .
Hope to see you there
Good Luck everyone on their exams !!!!


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PostPosted: Tue Nov 17, 2009 5:29 pm 
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thanks for the good wishes and for indication.
I know to ask too much ...... : can I read previous discussions about the A-GEM course ?
tks Francesco


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PostPosted: Tue Nov 17, 2009 5:34 pm 
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Meanwhile, I read the old chat, some are very useful to clarify some doubts.
tks again


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