What is the ruling on using natural materials such as lemon (used or new), spoiled milk, and yeast for skincare and beauty, while being careful not to be wasteful? Is this considered a sin?

The desire for beauty is permissible and even desirable for a woman, especially a married one, based on the Prophet's (peace be upon him) saying: "Indeed, Allah is beautiful and loves beauty." It is permissible to use foodstuffs like milk to beautify and care for the skin, as long as there is benefit in it and no extravagance or waste, and provided it is not placed in impure places after use. It is preferable to use non-food items if they yield the same benefit.