The pigment indigo is a natural dye that was extracted from the gland of the Phoenician sea snail (Murex brandaris). About 8000 snails were needed to produce 1 g dye. Naturally, the wool dyed with indigo was expensive. The dye was produced mainly at the Phoenician coast.
Exod 25:4; 26:1, 26:4, 26:31, 26:36; 35:6, 35:8, 35:23, 35:25, 35:35; 2 Sam 1:24; Prov 31:22; Song 3:10; 7:5; Jer 4:30; 10:9; Lam 4:5; Ezek 23:6; 27:6, 27:16, 27:24; Dan 5:7, 5:16, 5:29; Jonah 3:6; Nah 2:3; Sirach 40:4–5; 45:10; 1 Macc 4:23; Macc 8:14; Macc 14:43; Luke 16:19; Rev 17:4; 18:12, 18:16