Short answer: I recommend "viintaas golz" (shining/brilliant stone).

Longer answer: 

In Dovahzul you'll want to describe the crystal. I recommend not thinking in terms like this: "They went through the crystal portal", but like this: "They went through the portal of brilliant stone" (Nust bo zeim miraak do viintaas golz).

You can use the following terms to help you describe the crystal further: fiik (mirror), golz (stone), luft (face), 

Color descriptors: yuvon (gold, yellow), bii (blue), ved (black), sahqo(n) (red/crimson), graag (green), sadon (gray), sot (white). You can use these to help you describe various gems.