The most brilliant blue gemstone is undoubtedly zircon, which has a higher refractive index than even sapphire, tanzanite or spinel. Yet zircon is not well known by the general public, who are apt to confuse it with cubic zirconia, a synthetic diamond simulant. Zircon is a natural mineral called zirconium silicate that is found in a range of colors, including white, blue, yellow, orange, brown, rose and green.