Each mineral produces a unique diffraction pattern and can be identified from that pattern like a fingerprint. Identification of minerals is made by comparing their diffraction patterns with a library of over 17,000 mineral patterns stored in the International Centre for Diffraction Data (ICDD). Detection limits depend on the sample. For geological material, it is estimated that the minerals present in less than 2% of the sample might not be detected. The samples for X-ray diffraction analysis are ground or milled to a fine powder and then hand pressed into the sample holder. Approximately 1cm3 of the material is sufficient for rock mineral analysis but smaller amounts can also be accommodated by using a low background holder.