Evan Ludes of Omaha, Nebraska took these ice crystal/snowflake images.
"I had never seen iridescence inside an ice crystal until a couple days ago. Fresh snowfall brought some flakes of very interesting structure. When I took macro photos, I noticed that certain areas of the crystals seemed to have iridescent colors. The photos were taken with a Canon EOS 5D Mk1 and a reversed 18-55mm kit lens (@18mm, maximum magnification while in reverse macro). The crystals were lit by reflecting the light from a floodlight on our deck off them."
©Evan Ludes, shown with permission.
Each crystal, each snowflake, is different in its own way. Its form reflects its own, temperature and humidity history and journey through the cloud and to the ground.
Yet most have common features.
They have a hexagonal structure echoing the underlying molecular arrangement of oxygen and hydrogen atoms.
Water vapour molecules attaching to the growing crystal do so preferentially at incomplete layers or edges. The result, when growth is rapid, is a departure from the ideality of simple hexagonal prisms and plates into complex and individual shapes. Facets develop indents and dendrite arms sporting sub-growths extend from the crystal edges to give the multiplicity of snowflake expressions.
Ice colours come from trapped pigments, prismatic refraction and dispersion of light and more exotically from interference and birefringence effects.
Pure ice is almost colourless except in very thick layers.
Dieter Zawischa - visit his excellent "Causes of Colour"- explained the colours in these particular crystals:
"The colours in flakes (1) and (3) are due to interference. The rather dense succession of interference stripes seen at the upper right of the central hexagon of Snowflake (3) rules out birefringence as a cause, as birefringence of ice is weak, leading to much slower variation of colour. The conclusion is that there is a narrow hollow space in each flake producing interference as seen in 'Newton rings'."
"The colours seen in image (2) are different. They are certainly due to refraction and dispersion, these are spectral colours indeed. The paths of the light may be difficult to reconstruct, but from the colours, there is no doubt."