Open up Photoshop and create a new document with a white background. Really, it can be any size you want (within reason), but always remember that starting bigger and scaling down is much easier than starting smaller and scaling up. That being said, my document is 800 by 800 pixels.
Create a new layer and name it "Diamond."
Grab your pen tool and create the basic shape of a diamond which is fairly easy to make. If you take a look at a traditionally cut diamond from the side, it generally looks like a trapezoid on top of an upside down triangle, so here's what mine looks like. If you're unfamiliar with the pen tool, you might want to take a quick moment to look over my pen tool tutorial which will help you become familiar with it.
Now you just have to right click and fill your path with whatever color you want. At this point, it's not terribly important, but it helps us to lay the facets later (more below.) I'm just going to fill it with a fairly generic 50% gray.
When you think of a diamond, you probably think about a bunch of flat surfaces designed specifically to reflect lights. These little surfaces are called facets, and they're what gives diamonds their characteristic glittering appearance. To design facets, we're going to employ a few simple tricks that will give you a cool vector effect.
Create a new layer and name it "Facets."
Grab yourself your polygonal lasso tool and strap in! Here's where it's going to get fun. This is going to require a good eye and a lot of artistic liberty. The idea is to draw triangles on the surface of the diamond (we'll start with the top part, here) and use those to create a faux-reflective surface. Here's an example of a facet I've drawn. (Note: The white line is to define the top of the diamond from the bottom and will not be there in the final product.)
Now, grab your gradient tool and use a white-to-transparent fade. It should look like this:
Drag a gradient into your triangle so it looks like this:
Now you simply have to continue creating triangles across the surface of your diamond. Vary the gradient from top to bottom to make your diamond appear extremely shiny. Here is the finished top of the diamond.
Continue the facets on the bottom of the diamond, making sure to keep them elongated to follow the shape of the bottom of the diamond. Here's the finished diamond with all its shiny facets in place.
Of course, it doesn't have to be a diamond, you know. By changing your base color to another shade, you can take this image from a diamond to an emerald or a ruby in seconds.