Double refraction, also known as birefringence, is a property of some crystals of calcite.

Here's why:

* Calcite's Crystal Structure: Calcite has a unique crystal structure where the speed of light varies depending on the direction it travels through the crystal. This difference in speed causes the light to split into two rays, each polarized in a different direction.

* Other Birefringent Materials: While calcite is a classic example, other materials exhibit birefringence, including:

* Quartz

* Tourmaline

* Mica

* Some plastics

Let me know if you'd like to learn more about double refraction or calcite!