Historically, mosaic icons were made with traditional materials like smalti, marble, and gold leaf glass. Those traditional mosaic materials might be preferred if you are trying to make a reproduction that looks historically accurate, but they are more expensive and more difficult to work with.
If you have any latitude in choosing your materials, remember that it is possible to make striking and realistic images using ordinary vitreous glass mosaic tile, which is both affordable and easy to work with.
