making a 50:50 skill

i was coding a mob that is rare to find the player will have to look everywhere to find it then i was coding his skills i wanted that when player right click him he have a 50% chance of giving them a rare gem or a 50% get summoning evil guards to take out the player but so far i dont know how to make it happen i want it that one of the 2 skills will get applied once the player triggers it i know i can make it on 0.5 but sometimes it might endup with the 2 skill getting triggered at the same...

making a 50:50 skill