The Garnet Orb sits upon the Time Key Staff that Sailor Pluto wields in battle. When combined with the Time Key Staff, it is then called the Garnet Rod.
Pluto initially appears during the Sailor Moon R story arc having only the time key staff...it is unknown exactly when she gains the Garnet Orb, but she has access to it in the Sailor Moon S arc when all three Talismans would come to contact with each other.
The Garnet Orb alone allows Pluto to use "Chronos Typhoon" and "Garnet Ball".
The Garnet Rod allows for Pluto to use "Dead Scream" and "Dark Dome Close".