Astral Hailstones
Close to the heart of an astral storm, some of the raw thoughts and emotions that make up the storm may actually crystallize into solid form, forming astral hailstones. This phenomenon was noted by the early illithid colonists on the Astral when the Empire was in its heyday. A mind flayer explorer (probably a Creative) tasted one, and found the substance to its liking. Eventually, it became a popular recreational substance for many illithids of the late Empire. While they tasked some of their tougher servant creatures to harvest the stuff, they forbade their thralls from actually eating any of it.
After the Liberation, the consumption of Astral hailstones remained taboo among githyanki for many generations, in part out of habit and partly because the idea of "eating thoughts" sounded similar to "eating brains." The psion Ezad'ara was the first (famous) githyanki to break the taboo and popularize use of the substance; some scholars speculate that this fact may have had something to do with his being thrown out of his home city.
Astral hailstones, when consumed, have no effect on non-psionic beings beyond strong, strange impressions of alien thoughts and feelings. Those who are psionically aware who consume naturally occurring hailstones experience these thoughts and feelings more intensely, and may experience pleasure, a burst of mental energy, and/or hyperactivity (followed by fatigue and a tired mind a few hours later).
Ezad'ara discovered a means of 'refining' the substance via a mixed psionic/alchemical process, part of which involves a person of at least moderate telepathic ability sifting through naturally occurring hailstones, separating the most complex and interesting thoughts and the most overwhelming emotions from lower-quality chaff. These refined hailstones may actually contain fragments from multiple naturally occuring hailstones. The effect of 'refined' hailstones, on a psionically aware being, causes a powerful 'rush' and temporarily increases mental and psionic activity.
(In game terms, a psionically aware character who consumes refined Astral hail temporarily receives a +2 bonus to Intelligence and Charisma and a +2 bonus to Will saves for 2-3 hours. Additionally, if the consumer has levels in a psionic class and has fewer than 100% of his power points currently, the substance can restore up to 20% of them as long as the 'high' lasts. This is followed by a -4 penalty to Constitution and a -2 penalty to Will saves lasting 4-5 hours after the 'high' has worn off.)
Overdoses on refined hailstones can result in weakness, hallucinations and confusion, and severe overdoses... you don't want to have a severe overdose. Long-term, continuous use of the substance is also not recommended, and even a single dose can result in subtle changes in brain chemistry (a fact to which illithid performance-eaters can happily attest, as they tend to find such changes flavorful).
In githyanki society, the consumption of Astral hailstones has occasionally been banned outright in some fortress-cities, though in general, throughout most of githyanki history, the practice has been tolerated. (It was tolerated throughout the reign of Vlaakith CLVII.) Small doses of the least powerful hailstone fragments have made their way into some githyanki dishes, though by tradition, a would-be consumer must be informed of the presence of such an ingredient. The most refined hailstones are mostly only consumed by (and available to) Githyanki psions and some warlocks, usually for military purposes. Several of the psionic schools, however, have an artistic tradition/hobby of hyper-refining hailstones by selecting strong emotions and thoughts according to particular themes: a type of emotion, a specific range of emotions, thoughts pertaining to "war" from the viewpoints of multiple species, and so on. Some githyanki moralists compare this particular practice to performance-eating and consider it thoroughly disgusting!
The lowest-quality hailstones are usually available as a trade good to outside species (for those bold enough to trade with the githyanki in the first place), but the githyanki generally reserve the highest-quality hailstones for themselves. It's very hard for a graith to get ahold of the refined stuff, though a small underground trade in it does exist. Knights, members of the Ch'r'ai, and some seers often refuse to consume Astral hailstones altogether for moral reasons, though some other githyanki seers have a definite fondness for the stuff.
It is to be noted that the collection of Astral hailstones is, by its nature, dangerous and difficult, and thus they tend to be expensive. Young, noncomformist githyanki sometimes take up the profession of harvesting them.