There could be a few reasons why your white quartz is turning yellow. One possibility is that the quartz has been exposed to too much sunlight or heat, which can cause the mineral to change color over time. If your quartz is kept in an area with direct sunlight for an extended period of time, it may start to turn yellow. Additionally, if the quartz is placed near a heat source, like a fireplace or stovetop, it may also change color. Another possibility is that the quartz has been in contact with certain chemicals. Some acids, for example, can cause minerals like quartz to change color. If you have recently cleaned your quartz with a cleaning product that contains acid, this may be the reason why it is turning yellow. Finally, the color change could simply be a natural process that occurs over time. Quartz is a mineral that can slowly change color as it ages. If your quartz has been in your home for many years, it may be gradually turning yellow due to this natural process.