'Musawwamah' (marked) means: marked to indicate that they are stones of punishment, for upon each stone there was something like a seal.
﴿مسومة﴾ أَي: مُعْلَمَة أَنَّهَا من حِجَارَة الْعَذَاب، كَانَ فِي كل حجر مِنْهَا مثل الطابع.
