Hi Staudie,
The first place I would start your research is the mattress shopping tutorial here which has all the basic information, steps, and guidelines that can help you make the best possible choice … and know how and why to avoid the worst ones.
There is also more about the most important parts of the “value” of a mattress purchase in post #13 here. While Diamond makes some better than average quality/value mattresses compared to most of the larger manufacturers … outside of how suitable a mattress is for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) a mattress is only as good as its construction and the quality of the materials inside it (regardless of the name of the manufacturer).
You can see my thoughts about Diamond in post #2 here and in post #2 here. If you are considering a specific Diamond mattress then I would always make sure that you can find out all the information you would need to identify any weak links in that particular mattress and make more meaningful comparisons to other mattresses (see this article). Once you know the type and quality of the materials in a mattress then you are in a good position to make an informed buying decision. If the retailer that you are working with is willing and able to find out all the information you need then I would certainly consider them, but if they either can’t or won’t … then I would pass the mattress by (see the guidelines here).
I would also keep in mind that many wholesale manufacturers will only provide the specs of their mattresses to the retailers that sell them and not directly to the public and a good retailer should be willing to call their factory or their rep to find out any information you need about the mattresses they are selling. This “should” be part of their job and the service they provide.
Phoenix