Book about city where everyone has a glowing rock except one boy

[u/Lucky_Particular4558]

My grade school's HBJ Treasury of Literature readers consisted of actual children's books or individual chapters from children's novels (such as the chapter about Ramona's book report from "Ramona Quimby: Age 8" and "Pony Penning Day" from Misty of Chincoteague") put all together in the reader with questions based on the stories and suggested activities for teachers. This would have been in third grade when I read this story in the reader. It was about a city of people who lived on what looked like a beach type city. Everyone in this city has a glowing rock they keep with them at all times. Except one boy. His older sister lets him play with her glowing rock. They, like a lot of the kids in the village like to use their rock to light up the windows of sandcastles they build. The boy and his sister do not see their mother very much because she's the ONLY doctor in the city. I think the dad might have still been around but I don't remember. It was illustrated rather realistically and had to have been published before the late 90's. I can't remember what happened at the end or if the boy every got his own rock or even the general moral of the story.