Check out the Matted Cholla Cactus (Grusonia parishii). It also goes by Dead Cactus, Club Cholla, and (my personal favorite) Horse Crippler.
This low-growing cactus is a bona fide actual rare plant in California–a 2B.2 (Rare in California, but common elsewhere; moderately threatened in California).
For most of the year, the Horse Crippler looks gray and dead, but in the spring it develops reddish spines and even delicate yellow flowers. I only saw one bloom all summer–that’s rare in my book.
As an added bonus, here’s a photo of of a Desert Pincushion Cactus (Coryphantha chlorantha) in bloom. (California Rare Plant Rating 2B.1–Rare in California, but common elsewhere; seriously threatened in California).