By appointment only. Except if someone walks in ahead of you without one.
I needed a diamond appraisal and wanted to look at some wedding rings. I was a little confused in finding the place initially; it's on the third floor of Tivoli in a *shared* office space without any signage for D&R. The whole office appeared closed (lights off, doors locked, no one in sight). All these things struck me as a little odd for a reputable jewelry dealer during their operating hours. I called the owner and he let me in the locked front door and led me to a waiting room. He then apologized and told me that someone had just walked in ahead of me and that he would have them out soon to honor my appointment.
I gave up and let myself out after waiting patiently for 30 minutes beyond my appointment time without seeing or hearing from the owner again.
Diamanti Jewelers and Jeff White are both also in Tivoli, and my dealings with them have been professional and courteous. I'd recommend either of them before coming to this place.