This necklace as of August 1996 was being offered for sale for 1500 Galleons (HBP6), and was apparently purchased by or on behalf of Draco Malfoy at some time between August and October of that year (HBP12, HBP27).
The curse is apparently activated by direct contact with the necklace; while on display at Borgin and Burkes, the necklace was accompanied by a DO NOT TOUCH sign (CS4). Judging from its effects on Katie Bell, the curse causes the victim to levitate to a distance of about six feet off the ground – just far enough to complicate matters for anyone in a position to help – then begins to inflict dreadful torment. Whether this torment is physical, mental, or some combination of the two is unclear, but it is severe enough to cause violent convulsions and sufficient damage to require extended hospitalization after a few seconds’ exposure to the smallest possible area of skin. Touching it with one’s entire hand or actually putting the necklace on would cause sufficient exposure to kill the victim almost instantly (HBP12, HBP13).
Opals were once thought to have positive magical properties, but people started associating them with misfortune or death after Sir Walter Scott published his novel Anne of Geierstein, in which a magical opal talisman kills the weared after being exposed to holy water (source: Wikipedia). -BB