Silima was the substance invented by Fëanor to create the Silmarils. It means "Apt to Shine" in Quenya (from sil = "shine (white)" and -ima = adjectival suffix). As an adjective, its plural form is silimë.
The ingredients of silima were known only to Fëanor, though it was said to be made of the blended light of the Two Trees of Valinor. It suffused the crystalline structure of the jewels and gave them a radiance that shone even in the dark, yet gleamed more brightly when exposed to light.