Ruby should be worn on ring finger to get maximum benefit, not on index finger. In case of male it should be worn on right hand while in case of female it should be worn on left hand.
The reason why some people wear ruby on index finger is because Ruby and all Sapphires (Blue Sapphire, Yellow Saphhire, White Saphhire, etc.) are variety of corundum and normally Yellow Sapphire and White Sapphire is worn on Index finger that is why some people consider wearing ruby on their Index finger because Ruby and Sapphire have the same parent – Corundum.
But from astrological point of view this is incorrect. Ruby should be worn on ring finger.