Formula of ruby copper is:
A) ${\text{C}}{{\text{u}}_{\text{2}}}{\text{O}}$
B) ${\text{C}}{{\text{u}}_{\text{2}}}{\text{S}}$
C) ${\text{CuC}}{{\text{O}}_{\text{3}}} \cdot {\text{Cu}}{\left( {{\text{OH}}} \right)_{\text{2}}}$
D) ${\text{CuFe}}{{\text{S}}_{\text{2}}}$

Hint: Ruby copper is an important ore of copper. It is also known as red copper or cuprite. It is formed in the oxidised zone of copper sulphide deposits.

Complete answer:
Ruby copper is one of the most collectible and spectacular gemstones. It has a deep garnet colouring and higher brilliance than diamond. It is very soft in nature. It is dark red, cochineal red to black in colour. It has a cubic, octahedral and dodecahedral crystal structure. It is brittle in nature. It has adamantine, sub-metallic and earthy lustre.
Ruby copper is known as cuprous oxide.
The formula for cuprous oxide is ${\text{C}}{{\text{u}}_{\text{2}}}{\text{O}}$.
Thus, the formula for ruby copper is ${\text{C}}{{\text{u}}_{\text{2}}}{\text{O}}$.

Thus, the correct option is (A) ${\text{C}}{{\text{u}}_{\text{2}}}{\text{O}}$.

Additional Information: Ruby copper is very rarely used in jewellery. Ruby copper ores are found in Ural Mountains, Altai Mountains, Sardinia, Cornwall, France, Arizona, Chile, Bolivia and Namibia.
Cuprite dust is toxic in nature. If it is ingested accidentally it could lead to vomiting, liver damage and kidney damage.

Note: Ruby copper contains 88.8% of copper. Ruby copper is a copper oxide mineral. Ruby copper can be blackish red, crimson red or brownish red in colour depending on the impurities present. Cuprite is named as ruby copper because of its red colour. It was named by a geologist, mineralogist and physicist Wilhelm Karl von Haidinger.