Here is a list of the colors I use for Red Gems.. and that will be the base of this Tut but really it doesn't matter what color you use just pick three shades going from dark, mid to light and a highlight.
Mid Color - Scab Red or Gory Red (VGC)
Light Color - Blood Red (now Evil Sunz Scarlet) or Bloody Red (VGC)
Highlight - Golden Yellow or Golden Yellow (VAC) haha
White - Which ever you choose
I use inks a lot when working on gems as it gives an iridescent look with some transparency. I use a mix of old Citidel Inks that I still have left and Daler Rowney when I had to replace them.
Plasma Red (not pictured above)
Magenta (not pictured above)
It might look like a lot of colors but it really makes a difference and it doesn't take too long to give you a nice looking finished product.
**Click on the images below to enlarge if you are having trouble seeing the gem**
1. Start with Mech Red and paint the whole gem
2. Apply your light color to the bottom 2/3 of the gem. You also want to work at an angle, go diagonally from top left to bottom right.
Here is a diagram I whipped up to help show the directions I'm talking about ..