INCLUSIONS
Many crystals enclose a certain number of foreign particles during their growth. These inclusions often make a clear mineral appear cloudy, or may change the colour or some other characteristic of the mineral. Occasionally, gaseous or liquid remnants from the mother liquor are enclosed in the crystal. Quartz, for instance, may appear milky, due to minute liquid inclusions which are invisible to the naked eye. Other varieties of quartz, such as rock-crystal, may contain quite large, clearly visible crystals of actinolite, tourmaline or rutile.