In gemology, diffraction is the bending of light waves around an obstacle or sharp edge, either by transmission of light or reflection.
Colin Winter compares diffraction to water coming from a hose. It will travel in one direction. If you hold your thumb partly over the opening, the water will be dispersed in different directions.
Diffraction is the dispersion of white light into its spectral colors (red, orange, yellow, green, blue, indigo and violet).
When dispersion happens with a prism, the violet part of the spectrum is bent the most, but in diffraction, it is the red light that gets thrown the furthest. This happens because the longer the wavelength, the more the waves will diffract.
As the diffraction occurs in radiant waves, you will see a repetition of spectra along the center of the zero order spectrum(where little to no dispersion occurs).
The first order spectra(on both sides of the zero order) are the ones most clear, so it is a first order spectrum that is used in diffraction grating spectroscopes.
Diffraction grating material is made up of thousands of small grooves (or rulings) that run parallel over the transparent material, acting as slits.
This causes diffraction to occur on all those grooves. Due to interference, the different colors combine. The final result is seen as a series of spectra of white light after transmission through the grating material.
In opal, the spaces between silica spheres cause diffraction by reflection, and is partly responsible for the famous opalescence of precious opal.