I used jojoba oil on my belly while I was pregnant and I did not get a single stretch mark! I wish I had used it on my breasts!!!
I don't know about castor oil on the skin, but I did a bit of research about what I could use on my face and body( I'm pregnant and don't like to use un-natural chemical filled creams etc), and ended up purchasing organic jojoba oil. It contains natural collagen which is almost identical to that which our bodies produce, doesn't clog pours, is great for all skin types, can improve acne, great for wrinkles, is light and easily absorbed. On top of this, it is also great for the hair, helps resolve dandruff, and can be used to prevent/ treat stretch marks. All of this in the one oil sounded great to me, so I bought it and have been using it for 2 weeks, I love it. Apparently it has been used for hundreds of years as a wound/ skin healer.