"Iridescence" is the property of appearing different colors depending on the viewing angle. Iridescent Bricks in Terraria do not display this iridescence because they always stay the same color, even as the player moves. The shininess that these blocks do possess is technically described as "pearlescence", but this term was likely not chosen due to the similar name of Pearlstone Brick.
In real-life, iridescent tiles are decorative glass ceramics with a rainbow sheen that changes color (in a similar manner to oil slicks) while looked upon.
