What are sariras (scientifically seen)? What are sariras (Tibetan: ringsels)? 

I have seen those small crystal 'pearls' (exposed in bottles) at several (even small) monasteries in SE Asia. These relics remain (are found in the ashes) after the cremation of the body of (high appreciated) Buddhist monks.

Is there a scientific explanation for those sariras?
Wikipedia is referencing this article:

Scanning electron microscope observations of heat-treated human bone.
This report describes the heat-induced alterations in human bone tissue observed using scanning electron microscopy and microradiography. Femoral bone samples were taken from persons varying in age from 1 year to 97 years at the time of their death. The bone was heated at selected temperatures in the range 200-1600 degrees C for periods of 2, 12, 18 and 24 h. Macroscopically, changes in colour occurred, together with some shrinkage, fracturing and distortion. However, dramatic changes occurred at the ultrastructural level. These changes included the progressive combustion of the organic portion of the bone tissue up to 400 degrees C and recrystallisation of the bone mineral beginning at 600 degrees C. Recrystallisation produced a range of crystal morphologies: spherical, hexagonal, platelets, rosettes and irregular. Crystal growth occurred at temperatures &gt; 600 degrees C. Sintering led to fusion of crystals at 1000 degrees C. This process continued up to temperatures &gt; 1400 degrees C. At 1600 degrees C the bone mineral melted. On heating, the morphology of crystals formed, and the ultrastructural changes which occurred, were found to be related to the age of the deceased, the temperature to which the bone had been heated and the duration of heating. These results are of importance to forensic scientists, arson investigators and paleoarcheologists in their investigation of cremated human bones, particularly when only fragments of bone are available, in order to determine something of the life history of the deceased and the circumstances surrounding the death.

600 degrees C is the temperature at which material begins to glow red (i.e. an achievable temperature when a body is cremated), and 1000 degrees is when it's orange.