If you have a leaf, and want the viens embedded...you need to make leaf out of something. You mentioned POP.
If you took silicone and put it on your leaf, the mold would have the veins of the leaf embedded, and when you made your cement leaf from your mold, the veins would now stick up. It would look like the backside of your leaf.
to make your pop mold. Start small....maple leaf possibly...Make a small amt as it hardens quickly.....make a soupyier mix. Take a small artists paint brush and hold the leaf in your hand, Spray it lightly with PAM ---and brush POP mixture to the edges and lay the leaf down keeping the POP off the underside or front of the leaf and finish brushing the whole leaf with POP.. Without picking up the leaf as it may crack the first layer of POP.. now Brush on several more layers of a not so soupy POP mixture until it's about 3/8" - 1/2" thick. (you want your mold about as thick as you want your final cement leaf to be.) Not too thin, not too thick... Let it cure over night to make sure it is hardened very well.
Turn it over and carefully remove your leaf and all the veins that are stuck.. Thus a POP leaf.
Take a knife and clean up any edges etc....fill in tiny air holes....etc.
Now spray it with oil and make your silicon mold. The silicon mold can be washed up with warm water to remove all bits of POP. When you 'pour' your cement mold, be sure to lightly spray it first, and you can place something under it, to change the shape if your want. (Like lay it in a bowl for a bowl type shape or lay a pencil under it to lift the edge... If your leaf is small, you don't need cheese cloth!
Hope this helped!

Marcia

I have the leaf POP tutorial if you want to read it.
http://forums2.gardenweb.com/forums/load/hypertufa/msg0816290913843.html?7

What is POP?

Found my answer Plastic of Paris.
Cecil

...oooorrrr..... you can make a concrete leaf with the real leaf then make a silicone leaf mold from the concrete one!