[caryjames]

Hi folks I am getting ready to create some coloured diamonds and decided to check what value Maxwell used in it's diamond complex IOR file and I found a discrepancy between the refractive index real world value and the value that Maxwell used for it's diamond.

Real world ior- 2.417
Maxwell-3.00

Any thoughts? Maybe it does not matter but after some of the discussions around ior values for glass vs plastic etc I am a bit puzzled by the difference in these values.

The only natural gemstone with a refractive index of 3.0 is Prousite a gem too soft and too susceptible to natural light to be actually used as a gem in jewellery.

[Maximus3D]

The info i have in my IOR list is this.. maybe it helps.

Diamond ior value varies between 2,417 to 2,419.

DIAMOND

* Chemistry: C, Elemental Carbon
* Class: Native Elements
* Subclass: Non-metallics
* Group: Carbon
* Uses: usually as a gemstone and abrasive, also scientific uses.

Diamond is the ultimate gemstone, having few weaknesses and many strengths. It is well known that Diamond is the hardest substance found in nature, but few people realize that Diamond is four times harder than the next hardest natural mineral, corundum (sapphire and ruby). But even as hard as it is, it is not impervious. Diamond has four directions of cleavage, meaning that if it receives a sharp blow in one of these directions it will cleave, or split. A skilled diamond setter and/or jeweler will prevent any of these directions from being in a position to be struck while mounted in a jewelry piece.

As a gemstone, Diamond's single flaw (perfect cleavage) is far outdistanced by the sum of its positive qualities. It has a broad color range, high refraction, high dispersion or fire, very low reactivity to chemicals, rarity, and of course, extreme hardness and durability. Diamond is the April Birthstone.

In terms of it's physical properties, diamond is the ultimate mineral in several ways:

* Hardness: Diamond is a perfect "10", defining the top of the hardness scale.
* Clarity: Diamond is transparent over a larger range of wavelengths (from the ultraviolet into the far infrared) than is any other solid or liquid substance - nothing else even comes close.
* Thermal Conductivity: Diamond conducts heat better than anything - five times better than the second best element, Silver!
* Melting Point: Diamond has the highest melting point (3820 degrees Kelvin)
* Lattice Density: The atoms of Diamond are packed closer together than are the atoms of any other substance

Diamond is a polymorph of the element carbon. Graphite is another polymorph. The two share the same chemistry, carbon, but have very different structures and properties. Diamond is hard, Graphite is soft (the "lead" of a pencil). Diamond is an excellent electrical insulator, Graphite is a good conductor of electricity. Diamond is the ultimate abrasive, Graphite is a very good lubricant. Diamond is transparent, Graphite is opaque. Diamond crystallizes in the Isometric system and graphite crystallizes in the hexagonal system. Somewhat of a surprise is that at surface temperatures and pressures, Graphite is the stable form of carbon. In fact, all diamonds at or near the surface of the Earth are currently undergoing a transformation into Graphite. This reaction, fortunately, is extremely slow.

For natural diamond mineral specimens, see our diamond specimens page.
For loose (cut) diamonds or diamond jewelry, see Abazias Diamonds.

PHYSICAL CHARACTERISTICS:

* Color is variable and tends toward pale yellows, browns, grays, and also white, blue, black, reddish, greenish and colorless.
* Luster is adamantine to waxy.
* Transparency crystals are transparent to translucent in rough crystals.
* Crystal System is isometric; 4/m bar 3 2/m
* Crystal Habits include isometric forms such as cubes and octahedrons, twinning is also seen.
* Hardness is 10
* Specific Gravity is 3.5 (above average)
* Cleavage is perfect in 4 directions forming octahedrons.
* Fracture is conchoidal.
* Streak is white.
* Associated Minerals are limited to those found in kimberlite rock, an ultramafic igneous rock composed mostly of olivine.
* Other Characteristics: refractive index is 2.4 ( very high), dispersion is 0.044, fluorescent.
* Notable Occurrences include South Africa and other localities throughout Africa, India, Brazil, Russia, Australia, and Arkansas.
* Best Field Indicator is extreme hardness.

/ Max

[caryjames]

Hi Maximus- Thanks for that info.... actually I am a Gemmologist so I have that info . As an aside, if you do need any gemstone info I would be happy to dig through my reference books. I came across your COMPREHENSIVE ior list on the web last night- it is saved snugly on my hard drive

Do you have any idea why Maxwell would have the ior at 3.0 instead of 2.41?

[Maximus3D]

No problem oh so you're a gemologist, i didn't even know that jobtitle existed being a gemologist you should have tons of info about stones of all sorts. It's not often i get to render gemstones but when more info is always welcome.

That IOR list has grown a bit since i posted it on CGTalk way back. It contains even more precious info now

About Maxwell's 3.0 IOR, no i have no idea why it uses that. But i would stick with the real value instead of 3.0 which is way to high for diamonds. What you could do is use a scenesetup (photostudio type) and render a diamond there with 3.0 and then one with 2,417 and see the difference. See which one feels more real to you, my guess is 2,417 but you never know..

/ Max

[caryjames]

Thanks Maximus- I have been using the diamond ior from Maxwell and have not had any issues with it but after I finish this current project I am working on I am going to experiment with the different ior values- I would expect that 2.417 would be more realistic but so far my diamonds have looked fine.

I wish that there was an easier/more intuitive way of creating ior materials. It would be nice to be able to just plug in values for the following and be able to approximate a gemstones characteristics instead of having to work with a spreadsheet.

i.e:
refractive index
reflective index
dispersion
lustre

One can dream