According to the Strategy Guide I have... you have to 'bomb a wall' in Rolling Ridge... How helpful.
We unfortunately don't have a Rings guide yet, but our affiliate Zelda Universe does. Here is the transcript of the location. Hope it helps.
"Venture to Rolling Ridge in the present and enter where you originally found the Goron Elder back when he was trapped under the rubble. Wander around until you see the Goron pacing back and forth. Now bomb the east wall to open a way to the treasure chest containing this ring. With it equipped, every single item you pick up–whether Rupees, Recovery Hearts, Bombs, Arrows, anything–they all count as double. Once you find it, equip it right away (especially in the overworld), for this is by far one of the most valuable rings in the series, and it renders the Red and Blue Joy Rings completely superfluous."
When we do get around to a rings guide, we'll surely include maps and screenshots to make it o so easy.