Like the herringbone pieces in another thread these pieces with mica inclusions have been turning up for years and have recently erroneously been attributed to Bermondsey Art Glass. This might be because I loaned a marked example of that company to the exhibition ["Glassblowing in London" that was held at Gray's Antique Market by Circa Glass] which has mica inclusions within a blue casing, but there the similarity ceases.
I have compared the two types closely and the one that you are enquiring about does not exhibit the characteristics of Bermondsey.
Recently there was an unsubstantiated theory that the ones in question are by Powell from the 1930's, but I have not found any evidence that the theory is provable.
I am surprised that Steuben is being considered as an option, since I don't ever recall seeing a reference to that illustrious company producing pieces of transparent coloured glass encasing mica/silver inclusions. However, sometimes one can be surprised. Yet, if I am right, it would not be sensible to suggest that the piece is by them purely by reference to the shape. As I have pointed out before there needs a lot more evidence than just one characteristic before giving an attribution.