I'd assume it would be simple. Twilight easily became human, so I'd assume since Pony pinkie and Human pinkie are (relatively) the same entity, when switching places, they would also, in a sense, switch bodies
It happened during the EQG Special Spring Break. At the end, Sunset and the Girls lead all the passengers of the sunken cruiseship through Equestria back to the human world via Twilight's Portal Mirror.
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It may well be the case that they can't. If the portal turns ponies into (pastel) humans, why shouldn't it turn gemstones into something common on the other side of the portal, such as costume jewelry, and so on?