Our somewhat obscure mystery seed from last week was from an Arizona grape ivy, Cissus trifoliata.
It is called by other common names, such as cow-itch vine and sorrelvine. The name cow-itch comes from the fact that the leaves can cause skin irritation in some people.
Arizona grape ivy is a hardy vine that can tolerate the high summer temperatures. It is native throughout the southern United States.
The name "grape" ivy fits because it belongs to the same family as grapes, Vitaceae. The fruit are not edible, however.
Doesn't the seed look a little like a grape seed?