Extrinsic defects are from the presence of impurities, from atoms not contained in the normal stoichiometry of the pure crystal. These can be substitutional or interstitial. Even trace amounts of these impurities can radically alter macroscopic properties. When these impurities are purposely added, they are often called "dopants", and the process is called "doping". Non-crystalline compounds can also be doped.
Steel is an example of a doped crystal structure mentioned already. Another good example is aluminium oxide, also called alumina. This is a white power known for its strength and commonly used as an abrasive, such as in grinding tools, sandpaper, and toothpaste.
The most stable crystalline form of aluminium oxide is corundum. Pure corundum is transparent:
If corundum contains a fraction of Al3+ ions substituted for Cr3+, it will produce ruby:
Other colors of corundum are called sapphires. A blue sapphire is created by substituting a mixture of Fe2+ and Ti4+ ions into the place of Al3+:
Every mineral have its own importance when react with other so corundum when react with Alumina the color change to red or green
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