I'd guess it gets tricky if it's been "actively managed" during the marriage. Earnings could be considered marital. Especially if there was any (marital) cash contributions to the fund.
I've got a brokerage acct, an old company DRP - both pre-marital, no "marital funds" added to these accts during the duration, and not actively managed in any way - all passive. Agreed (thru mediation) these were pre-marital, and not to be split. But these were also all disclosed.