Start at the Royal Stable Master, as always...

Royal Stable Master says, "I see you have a Gemstone Horse voucher there. If you're going to act like money grows on trees, I can send you to someone similarly foolish."

You are sent to see Gimm the Jeweler, who is near the forge in Humberton.

Gimm the Jeweler says, "That [horse] over there? Yes, it's one of mine."

Gimm the Jeweler says, "In my travels I've seen many marvels, but the Gemstone Horse may be the most marvelous, and for someone in my line of work they are infinitely preferable to a more conventional steed. In the end I wasn't content to have only one, so I ended up trying my hand at raising them. It's going quite well, I would say, the only downside is the [upkeep]."

Gimm the Jeweler says, "The problem isn't how much they eat, it's what they eat. They don't eat grain or hay like other horses, they eat minerals. I used to have a pretty steady [supply] from the mines in the Black Mountains, but now..."

Gimm the Jeweler says, "With all the chaos lately that seems to have dried up. I haven't been able to find any good sources locally, but I have created an item that should help. Now all I need is an [assistant]."

Gimm the Jeweler says, "I need two bushels of silver ore and three pints of amethysts. This Seeking Stone is attuned to the minerals I need, and should take you to somewhere both can be readily found. Once you have the supplies you can use it again to return to me. It only has one charge of location and one of returning, so if you use it up or lose it you'll have to make it back under your own power."

/use the Jeweler's Seeking Stone that Gimm gives you, and you will be transported to the coast of Gwyddneau. Along the beach are rock objects that you can interact with - right-clicking on them collects the ore or amethyst. Wander along the coast (the objects are both east and west of where you port in) until you have two silver ore and three amethyst, then /use the Jeweler's Seeking Stone again and talk to Gimm.

Gimm the Jeweler says, "Wonderful, this should stretch my supplies for months. Fortunately for me the [Gemstone Horse] runs very swiftly but eats very slowly."

Gimm the Jeweler says, "Ah yes. Of course that's why you sought me out. Well you've proved yourself responsible, I'm sure you'll take good care of this beauty."

You then receive the Gemstone Horse.