Diamond

Diamond was an extremely hard gemstone found primarily in the cold, mountainous region of Faerûn and anywhere on Toril that had seen volcanic activity, either active or dormant. Nearly all probable diamond locations were deep underground, making them accessible only to dwarves and the other subterranean races.

Description
Gem quality diamonds were translucent to transparent and came in a variety of colors. The most valuable hues were clear (also described as blue-white), canary yellow, pink, brown, and deep blue. They were almost exclusively facet cut because, if done correctly, diamonds refracted ordinary light into beautiful flashes of color (known as "fire" in lapidary parlance). Diamonds were frequently worn for adornment.

Very few substances were harder than diamond and all were just as rare and scarce, or more so. Diamonds could be used to engrave almost anything and were prized for making fine cutting and piercing tools for artisans. A typical properly cut specimen had a base value of 5,000 gp.

Powers
Diamonds had an affinity for divination magic, and in the crafting of spells or items that improved vision or located objects. Diamonds were the first choice for gems of seeing and were required components for a helm of brilliance. This stone was also useful in making or destroying poisons. Depending on the process and magics involved, diamond dust could be used to create a deadly poison or be used to create items (added to an immersion) or spell scrolls (used in the ink) that neutralized poison. More broadly, diamond dust was a viable substitute in the making of magical inks when more appropriate substances were not available. In all cases, according to Elminster, magical workings involving diamond worked best when done at highsun.

A diamond jewel, placed directly against the skin of the head or neck, afforded the wearer some protection against spells of the enchantment/charm school and psionic influence or attack. Dreamless sleep could be achieved by wearing cut diamonds on at least two sides of the head.