I would recommend you store your rosin someplace cool. As it deteriorates, the appearance will start to change. If you live in a particularly warm area, you can keep your rosin in the refrigerator. Most cello rosin comes in a resealable container which should keep the product airtight, but you can always transfer your rosin into a glass jar like a mason jar that has an airtight seal. Rosin will naturally stick more to glass compared to something like silicone, but there's no chance of a chemical reaction between the material of your container and the rosin if you put it in glass.