True about x-ray diffraction data for set gypsum products:

The setting reaction takes time to complete and changes in the mixture begin as soon as the hemihydrate and water are mixed together.  X-ray diffraction data for set gypsum products indicate that there is less than 50% dihydrate present in Type IV and V stones, about 60% in Type II die materials, and over 90% in Type I plasters. Phillips Science of Dental Materials ed 12 pg 184