Correct Option (B)
Rubies and sapphires are both gem varieties of the mineral corundum. Chemically, corundum is aluminium oxide (Al₂O₃). The distinction in color between rubies and sapphires arises from the presence of different trace elements within the aluminium oxide crystal structure.
- Rubies owe their characteristic red color to chromium (Cr³⁺) impurities.
- Sapphires exhibit a range of colors, most commonly blue, which is typically attributed to the presence of iron (Fe²⁺) and titanium (Ti⁴⁺) impurities. Other trace elements can produce sapphires of different hues.
Despite their varied appearances, both gemstones share the same fundamental chemical composition of aluminium oxide.
Incorrect Options
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Silicon dioxide (A): This compound is the primary constituent of quartz, a common mineral distinct from corundum, and is not the chemical basis for rubies or sapphires.
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Lead tetroxide (C): Also known as red lead, this compound is primarily utilized as a pigment in paints and in the manufacturing of batteries. It is not a gemstone.
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Boron nitride (D): Boron nitride (BN) is a synthetic compound known for its extreme hardness and thermal stability, often used in abrasives and cutting tools. It does not occur naturally as a gemstone like ruby or sapphire.