When the name is written Wolkstien, it means literally translated "cloud stone." 'Wolk' is Dutch for 'cloud', and 'stien' is a different written form of German 'stein', which means 'stone.'
When the name is written Volkstien, it means literally translated 'people stone.' You're indeed right: 'Volk' means 'folk, people.' With this form of the surname, I'd think you're right about the theory of 'stone people.' Also it could be some nickname for an unfriendly, cold of folk/people; ever heard the saying 'as cold as stone'? But I think your theory is more likely.
But with the Wolkstien surname, I doubt if it's got to do with stones somehow, unless it's a differently written version of the Volkstien surname. I can't really say something about Wolkstein, sorry.