Posted: Jul 9 2013 on LTS forum by Christopher Azzopordi:  

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"I'm photographing all my collection of seashells. I had a problem photographing the glossy ones, such cowries. Coming from a photographing background I did not like the hot spots (shiny part) so I built a lighting tent. However, the seashells now lack that lustre effect. What do you think is the best way with or without the hotspots?"

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Marlo: 

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​Haven't figured out how to have it both ways. Luster results from the interaction of light with the shell surface and is a function first of its reflectivity, and secondly the intensity of the light source. The greater the reflectivity and/or light source the more brilliant the reflection (luster). "Hot spots" (or glare spots) are those spots on reflective surfaces where the line of sight is along the direct angle between the camera lens and the glare source (light). The dimmer or more diffuse the glare source, the less intense the glare spots and overall diminished reflection (luster).

To capture the luster, it seems the hot spots are unavoidable. Eliminate the hot spots by diminishing the light and the luster diminishes with reflectivity.

Here's one rather difficult solution that partially works. Instead of diffusing all the light shining on the shell, cut a piece of diffusing material corresponding to the glare spot and place it between the light source and the shell (lots of experimentation required to get the right distances) so that only the light hitting the hot spot is diffused. Takes a lot of patience and ingenuity, but when successful it can give very passable results. I've had success, but gave it up as too arduous for the payoff.