Small crystals also known as rhoksial crystal are components to the wizard spell Heezlewurst's Elemental Buffer. Each crystal protects from something different.
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:
You can obtain the small string bag in the following locations:
- Mit-sin-bibh-khot's world of rare reagents
- Stapleton's Student Supplies
- The Guild of Alchemists' supply shop
The fact that they are sold together means it is less convenient if you just want one type.
See also Kefka's itemdb.