Answer:
What is easy to get into, but hard to get out of?
I am black within and red without With four corners round about.
I can be simple or I can be complex. I can be found in this riddle or in everyday life. I can be shapes or even colors.
I am used when you jump off a bridge for fun.
I can never be stolen from you. I am owned by everyone. Some have more some have less.
Though not a plant, has leaves. Though not a beast, has spine. Though many wouldn't need this thing, It's more valuable than wine.