From Discworld MUD Wiki
Small crystals also known as rhoksial crystal are components to the wizard spell Heezlewurst's Elemental Buffer and the long white wand. Each crystal allows protection or dealing a different damage type.
The small green crystal is also a component to the wizard spell Professor Flambardie's Grim Amulet.
They come in random quantities and colours in a small string bag in several colours
The small crystals are sold in a small string bag in the following locations:
- Mit-sin-bibh-khot's world of rare reagents in the Institute of Illusory Learning
- Stapleton's Student Supplies in the Artificers' Academy
- The Guild of Alchemists' supply shop
- The Creel Springs House of Magic shop
- Small, dingy shop selling magic items on the Serpent, in Bes Pelargic
- Component shop within Bes Pelargic L-Space Bubble guild
- Strange alleyway, a magic stall in Ephebe
The fact that they are sold together means it is less convenient if you just want one type.
"Hedge Wizard" NPCs that wander terrains (usually around Ramtops and Uberwald) sometimes have them, too, as Hedgies need protection from the elements!
See also Kefka's itemdb.