In the original PlayStation 2 versions, the Ruby Ring is bought for 8,500 gil from Mosphoran Highwaste (can be visited after Tomb of Raithwall events), Mt Bur-Omisace (after defeating Judge Bergan), and Phon Coast. It is found in a treasure in the Ozmone Plain's Field of Fallen Wings (60% chance to appear, 33% chance for gil, 10% chance the item treasure is a Ruby Ring with the Diamond Armlet equipped ). It can be stolen from Judge Bergan (10%).
In the updated Zodiac versions, the Ruby Ring is not available in shops, but can be sold for 4,000 gil. It is a treasure in the Stilshrine of Miriam's Walk of Prescience (100% chance to appear), Pharos - First Ascent's They Who Thirst Not, and Subterra's Abyssal - North. It is also found in Trial Mode stages 78 and 91 with the Diamond Armlet equipped. It can still be stolen from Judge Bergan (10%).
The Ruby Ring is also a reward in the Hunt Club sidequest on the Phon Coast. It appears in the Shifty-Eyed Merchant's store for 10,200 gil after the player gives Stok five trophies from hunting Rare Game.
The accessory could be useful where the player is bouncing magick off their party onto enemies; if all allies have Reflect, and the player reflects an area magick, it will bounce off from every party member and multi-cast onto opponents. This is useful against some tough marks like Wyvern Lord and White Mousse. The drawback is that the player cannot heal with magick without an Opal Ring on the caster (which they can't equip simultaneously with Ruby Ring). However, since the status will lapse as soon as the player removes the accessory, and the menu is always available, even this is not a huge hindrance.
Another good use is simply against enemies that use a lot of magick that can be reflected, though many of the enemies with the strongest magicks use piercing magick that cannot be bounced with Reflect.
The Mirror Mail also bestows a constant Reflect and so can be used instead of the Reflect Ring. The player can also set a Self:Reflect or Name:Reflect or Ally:Reflectga gambit to keep the effect active, but it will require some time and MP.