The focal length of a macro lens only affects the working distance from the object. Both can go to 1:1 magnification. If you need to be 6" from a stone with a 50mm lens, the 100mm will give you another 6" of working distance.
Are you using portable small reflectors to highlight facets on the gemstones? If you aren't you should be doing this in order to emphasize the 3D nature of the stone.
For my jewelry photography I use a light tent with 100wt daylight balanced fluorescent lights on either side. With a raised piece of frosted glass above colored paper or black velvet I can control shadows and background color.

One consideration to put into the equation.....

Do you have any illusions (and I do consider it an illusion as opposed to a good idea) to moving into A Canon Full Frame camera, you should consider the extra expense of going with EF lenses instead of EF-s.

EF-s lenses will not give you the full capability of a full frame sensor if that is your upgrade path. They are crop factor lenses designed for the smaller APS sensor in most of the Canon mid level bodies.

However, If the budget and future plans preclude full frame for any reason, I would not shy from refurb or used lenses from good sellers. I will even consider an eBay seller and have often when they have lots of sales and feedback at least 99% positive. I will NOT buy from someone on eBay with 96, 97, 0r 98 % positive feedback.

Good sellers are KEH, B&H, Adorama. I shy from Amazon because most sales on Amazon are pass through sales from average sellers, and the feedback system on Amazon is not that great. I will by new from Amazon if Amazon is the actuall seller, direct to the buyer (you).

Just a couple of considerations.

On refurbs, stick with factory refurbs, not just seller refurbs. Remember that refurbs are checked out, perhaps repaired, and come with some warranty from the mfr. Seller refurb generally means, " Hey we took a picture with it and we recognize the subject shot" Stick with factory refurbs.

It is not the color of the person! It is the brains they have in their head, the love of people, the want to have everyone be equal, and the love and belief in God. He doesn't have any of these!!!! He is out of control with every word that comes out of his mouth. And I really think he is a puppet for others doing the thinking for him. You have heard about brain washed folks carrying bombs around and blowing up things. That is the president in my eyes. He is brain washed and has no clue what he is doing. God help us all!!!!
Erv

I realize that, but if you happened to see a deer 50 foot away, you would not be able to capture an image of it without first removing the tubes...With a standard macro lens without tubes, your depth of focus is not limited. You can shoot an insect at life size & simply turn the focus ring to focus on a subject much farther away....Can't do that if you have tubes on the lens. With tubes, changing the focus distance on the lens will change the magnification ratio somewhat, but nowhere near what the macro lens alone will...