The purple color comes from manganese added to the glass when it was made. Manganese was used to actually make the glass clear, and used until about 1916. The purple hue comes from the interaction of UV radiation from the sun and the manganese. This piece was most likely originally clear, but turned purple from being in the sun for 100 years....
At first I was thinking it might be the top to an inkwell but now it seems to me that the scale is too large. Hard to say what the overall shape is from such a small piece.
Comments
Jeff
For the amethyst tinted piece, the diameter of the opening would probably be the best bit of info to imagine the overall size of the piece.