Emeralds

The Slytherin hourglass in the Hogwarts Entrance Hall for recording House Points is filled with green emeralds (PS15, Pm).

• The eyes of the entwined snakes on the door of Chamber of Secrets were made of "great, glinting emeralds" (CS16).
• Madam Trelawney wore emerald earrings during Harry's first Divination class (PA6)
• The locket of Salazar Slytherin, which was turned into a Horcrux by Lord Voldedmort, was decorated with a "serpentine S" formed of many small green stones that "glinted dully"  (DH14, DH19).
• During the Battle of Hogwarts, the Slytherin House hourglass was shattered, and the battling students and Death Eaters alike slipped and fell due to the emeralds all over the Entrance Hall floor (DH32).

Many things in the books are described as being "emerald green" in color.

• Minerva McGonagall's cloak on Privet Drive, and her dress on Sorting night (PS1, PS7)
• Harry's eyes are described as "bright green," just like his mother Lily (PS2, PS12).
• The ink on Harry's Hogwarts Acceptance Letter (PS3, PS4)
• Harry's first Christmas sweater from Molly Weasley (PS12)
• Dobby the house-elf has "enormous green eyes" (CS1)
• The flames produced by Floo powder (CS4, OP29, OP36, OP37, HBP1)
• The suits of the Ministry car chauffeurs (PA5)
• Voldemort's Dark Mark in the sky is made up of emerald stars (GF9)
• The lawns on Privet Drive before they were dried up by a drought (OP1)
• The shawl of Emmeline Vance, a member of the Order Advance Guard (OP3)
• Voldemort's Cave Potion that killed Regulus Black and poisoned Kreacher and Dumbledore was described as "emerald liquid emitting (a) phosphorescent glow" (HBP26)
• Horace Slughorn's silk pajamas in which he duels Voldemort during the Battle of Hogwarts (DH36).