Explanation:

- Sapphire and ruby are gemstones known for their beautiful colors.

- Both are varieties of the mineral corundum.

- Corundum is primarily composed of aluminium oxide.

- Sapphire comes in various colors, typically due to the presence of trace elements.

- Ruby is red due to chromium impurities in the aluminium oxide.

- Option 1: Aluminium sulphide

- Not relevant here, as sapphire and ruby are not made of this compound.

- Option 2: Aluminium hydroxide

- Incorrect, since this compound is not part of sapphire or ruby.

- Option 3: Aluminium carbonate

- Unrelated, as this compound does not form sapphires or rubies.

- Option 4: Aluminium oxide

- Correct answer, since both sapphire and ruby are forms of corundum, which is essentially aluminium oxide.