It could be. Purpleheart will turn brown as it ages. try setting it in the sun for a couple of hours. I have done it and had the purple return very vividly even after a finish is put on. Even if it don't, very nice goblet.
Try the sun trick and see what happens. If it doesn't turn purple, my money is on mahogany. At least that was my first reaction as soon as I saw it. 8)