REAL horchata is obtained by crushing chufas, leaving them in water
and sugar for about 24 hours and straining the mixture.
Chufas, cyperaceae cyperus esculentus as they go by their scientific
name, are a chick-pea sized tuberous roots of a sedge-like African
plant, with a brown skin and white flesh that I have never seen in any
other places than in Spanish or Mexican markets.
Rice or almond horchatas are only variants, and not even half as good as
the horchata de chufas.
From: Mathieu Wehrung in rec.food.cooking on Jul 24, 1996.