Large single crystals of CD 4, 4He, and 3He have been grown from the melt at low temperatures. These “quantum” crystals were characterized with X-ray diffraction techniques which allow excellent determinations of crystal quality. The crystal CD 4 was grown from its melt at saturated vapor pressure. Over 100 helium crystals were grown at constant pressures between 2.9 and 13.8 MPa. It has been found that the hcp and bcc crystals grow in preferred directions, and that growth rates of about 0.3 mm/ min are appropriate for hcp 4He, while bcc 3He requires the slower rate of 0.06 mm/min for good quality crystals. There is some indication that it is the isotope, and not the crystal structure, which is the cause of the fact that the hcp 4He crystals are much more stable than the bcc 3He crystals. Comparison of several theoretical crystal growth parameters for many substances leads to the conclusion that methane is basically a normal material, while helium is abnormal, having very different growth parameters from most substances. Helium is interesting in that its crystal growth parameters can be changed by very large amounts while keeping the chemistry constant.