Dark Amethyst in Baldur's Gate 3

Dark Amethyst found in the Whispering Depth. You can get to it through the blacksmith's house or the well located in the center of the Blighted Village.

Move through the Whispering Depth and search the Apprentice's corpse. Read the book to start the Search the Basement quest.

In the place where you will encounter the phase spider matriarch, on the lower level (where the regular spiders will be), look for a purple item. This is a dark amethyst.

What is it for? Head to the house opposite the forge in the Blighted Village and find a hatch that allows you to go down. In the basement, you need to go through several rooms, open secret doors and find a grate at the end. Open the grate to reveal Thai's Necromancy book. You can try to read it (save in advance and pass the checks). But you won't be able to read the last page until you're in Baldur's Gate. The alternative is to drop the book on the floor and cast any radiant damage spell. The quest will end. But if you study the book, you will unlock the Speak with the Dead skill. However, you can get it in other ways.