What materials are permissible for Istijmar besides stones?

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It is permissible to use objects that serve the same function as stones, provided they are cleansing. This general principle applies because the underlying reason (ma'na) for using stones is the removal of the physical impurity ('ayn al-najasa), which can be achieved by other means that cleanse effectively. What is used must be purifying; materials that are slippery or non-cleansing, such as glass, soft charcoal, or the like, are not sufficient because they fail to achieve the intended purification.