Fluorescence in Diamonds

 

Photo credit: Svetozar Simov

 

 

(This is an archive of an excellent post in our Fluorescent Mineral Facebook group by Richard David Armstrong)

 

There have been a couple posts on diamonds the past couple days, so thought I'd give a quick explanation of fluorescence in diamonds. Fluorescence in diamonds is caused by structural defects. Diamond is pure carbon and has only two primary trace elements, nitrogen and boron. Both create defects in the crystal lattice. Based on these impurities there are 4 primary diamond types. Type I contains nitrogen. Type II contains no detectable nitrogen. Type I is divided into type Ia which has groups of nitrogen atoms replacing carbon. The nitrogen can be in the form of N3 centers which is a group of 3 nitrogen atoms. Then there are A and B aggregates. A aggregate is 2 nitrogen atoms replacing carbon. B aggregate is 3 nitrogen atoms with a vacancy. There are 2 subtypes in Type Ia, Type IaA and IaB. You can also find Type IaAB which are probably more common. Type Ib contains single isolated nitrogen atoms.

 

Type II diamonds are divided into Type IIa which is the purest type of diamond with no measurable nitrogen, just pure carbon. Type IIb contains boron and are gray or blue in color depending on the amount of boron. The Hope diamond is a Type IIb.

Type Ia diamonds make up about 98 percent of all diamonds found. Type Ib about 1 percent, Type II about 1 percent with Type IIb about 0.1 - 0.2 percent.

 

Nitrogen defects are the most common cause of fluorescence:

  • Blue fluorescence is caused by N3 centers. They will generally show a 415nm absorption line in the spectrum. Blue fluorescence is the most common and is only in Type Ia diamonds.  Green fuorescence is caused by H3 or H4 centers. H3 is an A aggregate trapped by a vacancy. H4 is a B aggregate (3 N trapped by a vacancy in Type Ia. They will have 503 o4 496 nm absorption line.

  • Yellow fluorescence is caused by carbon structures called platelets or hydrogen impurities in type Ia.

  • Orangey-yellow is caused by single N atoms replacing C.

  • Orange is caused by a NV center with no charge (nitrogen trapped by a vacancy)

  • Pink/red is caused by a NV- center (NV with a negative charge).

  • Red is caused by boron in Type IIb diamonds. I may also cause red phosphorescence.

  • Type IIb diamonds may show a weak blue fluorescence even though they show no measurable N.

  • High levels of A aggregates can quench fluorescence. Since Type IaA and Type IaAB are the most common I think this is why only about one third of all diamonds will be fluorescent.

 

There are other causes that are not well understood. For example Chameleon diamonds which show color change from mild heat or storage in the dark can fluoresce intense yellow in LW, faint yellow in SW and phosphorescence for 30 to 60 seconds after exposure to SW. Mixed types of diamonds are common and this may affect fluorescence as well.

 

Natural diamonds generally fluoresce with exposure to LW and may be weak or inert in SW. Lab created (synthetic) diamonds generally fluoresce with exposure to SW and may be weaker or inert in LW. Some HPHT Lab diamonds will not fluoresce under either LW or SW but after exposure to SW may have a weak greenish phosphorescence for 10 to 30 seconds.

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