B&H Photo is a great online store to buy electronics; very similar to Newegg, and I am a B&H customer for over 25 years and recommend them. Local stores such as Best Buy, Walmart, Sam's Cluc, Costco, are just as good but maybe priced a little higher.
After you start out with the Nikon D3100 and get familiar with Manual settings as well as Automatic settings, look into a "Macro lens". A Macro lens is what you really need for close-up photography of items like jewelry, stamps, collectibles, etc. - This Nikon 40mm f/2.8G AF-S DX Micro-Nikkor Lens will serve your needs - http/www.bhphotovideo.com/c/product/810414-USA/Nikon_2200_40_mm_f_2_8G_AF_S.html
When you use a Macro lens a rigid tripod is highly recommended.
For online camera purchases make sure you buy "USA" and not "Imported" - Imported means that the product is from the "Grey Market" and does not have warranties and therefore will be cheaper. I do not recommend 'Imported'.
any camera which allows for interchangeable lens should do. i use the canon so my knowledge is limited to it. canon 60D is a good camera and if you are photographing jewelry you would most definitely require a macro lens. 100mm or more. get the L lens if you have the budget.
get yourself familiarized with focus stacking to get crisp clear & sharp photographs. sorry if this is too much info.