I've done sand tray in all different types of trays with all different types of sand. It really is magical. When I'm stuck, it gets me unstuck. When I can't verbalize something, I can put it in the tray instead. It really does work best if you don't overthink it and just go with wherever your mind goes. I used to feel a little silly, but now I choose to go to the sand tray often. When I sit down and arrange the figures, I usually have no idea what I'm really showing, so I usually leave amazed at how much I did. It's deep work. I have done it with therapists who are highly trained, but I've also done it with some who aren't. Ask your therapist if your interested. I think a lot of them know something about sand tray and it really is awesome if you are willing to go there.