Venom Rubies are most useful for at least one Opportunity card in the Lapidary storyline that needs them. They may be used for other cards, I'm sure.
Fortunately, you can keep running shroom races until you get as many as you want...winning the race also rewards you handsomely for a number of different items (Glim, Moon-Pearls, Jade, Rostygold, and probably more).