1.) I'm currently taking pictures or really expensive and really high carat diamonds (like 3-4 carat weight). I'm having a tough time with the reflections coming off the diamond making it look not pure. Is there any way to filter out the light from the flash? Someone suggested that I use a polarized filter but I honestly have no idea how to go about using one of those.
Warning: Inbound Physics Lesson Light is a wave. Picture it like the sea. Except that unlike the sea, there are lots of waves, all going in different alignments. Some are bouncing up-and-down, some are left-and-right, etc. But when light reflects off a surface, all of the light waves are bouncing in the same alignment. A polarising filter works as if it has slots, that only let through light waves bouncing in certain alignments. It sits on the front of the camera, and you can rotate it relative to the camera lens. By doing this, you can then align the 'slots' against the 'bouncing' and cut out the reflections.
This may not work with a multifaceted diamond however, since you could get an alignment of bouncing from each face. But it's worth a try.
2.) I want to take pictures that mimic the style of Top Gear's filming, that so of high contrast, gray gradient-ish looking feel to it. Would that be something I do with the camera or in photoshop afterwards? I've found gray gradient filters but I'm not sure what that actually does.
Thanks in advance!
Top Gear are using gradient neutral density and polarising filters a lot. A ND filter (also sometimes called a grey) just cuts out light coming into the lens. If you have a part of the filter that is grey, and a part that is not, (a grey grad) you can then omit light from a part of the frame, according to how you mount the filter. There are hard cutoffs and fades to be had at the border between grey and not grey, and these are called "hard" and "soft" accordingly.
To be honest, I do most of my ND grads in Photoshop. I would like a couple of non-grad NDs in my bag so that I can drag the shutter at times, but I haven't got around to it yet.
Polarisers, in addition to manipulating reflections (above), boost the saturation of images, particularly in some alignments against the sky. It's possible to get the sky to go a deep blue and the clouds fluffy white. Both of these effects can be simulated in post-processing, but you would often spend 20-30 minutes doing this in post, but only 30s putting a polariser on the front of your camera, and a properly polarised image always looks better than a pp-simulated polariser.
You should definitely have a Circular Polariser (CPL) in your bag for your walkaround and your wide angle lenses. Whilst you can get a 58mm CPL for around £20, it's worth spending sensible money (£70-ish, more for bigger diameters) to get better IQ.
HTH, feel free to +Rep me if it does.
[edit]Here are a couple of shots where I simulated CPL. As you can see, not very convincingly...
And one with simulated CPL + ND Grad
And one with just a simulated ND Grad
