A few of world’s treasured ornamental stones like pearls, corals are plant and animal products. But majority of them are precious or semi precious stones called gems are minerals. Some minerals are invariably the same color. For instance Malachite is always green, azurite is always blue and realgar used in making fireworks is red. But many occur in a variety of colors, resulting from mixing with other minerals, chemical impurities, and other causes.
Quartz is typically as clear as glass. Traces of titanium however transform it into rose quartz, while small amounts of iron result in the purple variety known as amethyst. Brownish smoky quartz, in contrast, is believed to be the result of natural radiation. Impurities have transformed colorless mineral corundum into precious gems. Sapphires are crystals of corundum tinted with titanium; rubies also are corundum crystals, colored with traces of chromium. Emeralds and aquamarines are simply differently colored forms of the mineral beryl, made precious by the presence of impurities.