Birthstones of the Days of the Week  Sun Mon Tue Wed Thu Fri Sat Topaz Pearl Ruby Amethyst Sapphire Emerald Turquoise Sunstone Moonstone Star Sapphire Star Ruby Cat's Eye Alexandrite Labradorite The second row of gem stones have optical effects. Some of them show a star (called "asterism" caused by needle-like crystal trapped inside), while others display different colours under different light source (e.g. alexandrite shows red in incandescent light but green under sunlight).