Unlike wedding shooter Sam Hurd who uses equilateral prism made of glass I normally use different types off crystals to get that experimental camera feeling. My opinion is that in some situations you are clearly allowed to let us say “breathe life into your images” without using an digital effect.
This technique results in a variety of effects from rainbows to different light-leak shapes. The incidence of the illumination is also very important producing entirely organic effects changing from frame to frame, and almost impossible to repeat or create in digital postprocessing.
Also why using one dollar glass junk in front of a thousand dollar lens… actually modern glasses are very resistant in getting light flares… well this technique is similar to the foreground bokeh technique, simply put something shiny in front of the lens, illuminate it and play with the focus and the distances.
When positioning the crystal in front of the camera, think about the direction of your light sources. You can achieve light-leak effects when the light comes toward the camera, while reflections are generally more interesting with strong side light.
Actually this is nothing new in the analogue era there were a huge array off effect filters… my favorite was Cokin serie A A055 Star… crazy one… the Cokin serie P p084 Difusor… fog filter… Orion spectral effect filter… and… and…
Shot with the 5D Mark I and the Zeiss 50mm f/1.4 ZE Planar T* in an Romanesque architecture church… not easy to focus and handling with a crystal in front of the lens. Edited in Lightroom with VSCO presets.
Although it looks easy there are some issues that might ruin a shot, personally I try to move the crystal in order to get the face almost free of lights and well that saved me of getting crazy in postproduction and of course I try not to abuse of it… when you are handling the crystal in front of the camera don´t forget the direction of your light sources… this is quite important…
I think the easiest way in order to understand the behavior of similar effects might be the use of christmas lights.
Nothing more… here another example with this technique.