I would start with the Conjuring (Thaumaturgy) rules, with the advantage that manipulating an existing object makes the final product permanent (not Ectoplasm), and thereby doesn't require a duration (or it requires a several-shift premium which we will call duration), but crossing thresholds isn't a risk for it. Though that may set an unwise precedent that matter can be easily disposed of by transmuting it to other matter.
Here are the guidelines I came up with:
http://www.jimbutcheronline.com/bb/index.php/topic,23613.0.htmlOf especial importance would be the Modifiers section, about believability and complexity. The GM should determine how plot-powerful a given change would be, then add Complexity as desired. Also, rapidly altering a mundane object with moving parts should be just as hard - or even harder - than making one up out of Ectoplasm.
That said, all of this goes back to the lead-into-gold issue. With this power, why isn't the character taking dirt and other detritus and turning it into gold?
As for the Evocation option: if the GM allows it, then start with an Evocation Maneuver to place the Aspect "Malleable Form" or something, then use the free tag to Invoke for Effect: the GM can charge extra Fate Points if a desired Effect would require more effort.