The rainbow is up to 7.6 cm (3 inches) long , and is elongate and oblong in shape. The shell is usually fairly thin. Males are compressed and females are inflated. The anterior end is uniformly rounded, the posterior end sharply rounded in females to bluntly pointed in males. The dorsal margin is straight and the ventral margin is straight to gently curved.
Umbos are low, even, or raised slightly above the hinge line. The beak sculpture has four to six double-looped ridges, the first two or three concentric. The umbos also have tubercles at the posterior end.
The periostracum (outer shell layer) is smooth except for growth lines. The shell is yellow to yellow-green with heavy broken green rays. The rays are more numerous on the posterior two-thirds of the shell.
On the inner shell, the left valve has two pseudocardinal teeth, which are small, erect, and divergent and sharp-pointed. The two lateral teeth are straight, short and fine. The right valve has one erect, columnar pseudocardinal tooth. Anterior to this tooth sometimes is a smaller nacreous swelling. The one lateral tooth has short and thin.
The beak cavity is shallow. The nacre is bluish-white, more blue posteriorly, and beak cavity is cream-colored. The posterior end is iridescent.
In Michigan, the rainbow can be confused with the ellipse and the rayed bean. The rays on the ellipse are fine, wavy and generally unbroken. The rainbow also has a longer hinge line and has finer teeth. The rayed bean is smaller and generally darker in color and more inflated.
Range length: 7.6 (high) cm.
Other Physical Features: ectothermic ; heterothermic ; bilateral symmetry
Sexual Dimorphism: sexes shaped differently