The only difference between "plaster" and "quartz" is the addition of quartz aggregate. The base components of "plaster" are sand, white cement and water. The base components of "quartz" are quartz, white cement, water, and sand. Where the difference comes in is the ratio of quartz to sand. This ratio can vary from 99% quartz and 1% sand to 1% quartz and 99% sand (yes I have see contractors put one 8 oz scoop of quartz in a mix and call it a quartz pool). I prefer a 50% to 50% mix which offers good work-ability of material and increased strength. When comparing always ask what the ratio is, you may have two bids for quartz with thousands of dollars difference. One builder may use 20% and the other 80%. Quartz is the stronger of the two finishes and the addition of color quartz can cosmetically change the look of the pool surface and the water color. The thing to remember is maintenance is the key to how long the pool will last. Many people can get 20 plus years out of plaster and many can only get 15 out of quartz. The difference is maintenance.