Sheikh Sulaymân al-Tuwayjirî
It is recommended that the grave should not be raised more than one hand span. It should also be demarcated with some sort of ridge. The purpose for this is to make it clear that it is a grave so it will be respected.
Jâbir has narrated that the Prophet (peace be upon him) raised the level of a grave one hand span.
Al-Qâsim b. Muhammad said to `Aishah: “O `Aishah, mother, show me the grave of the Messenger of Allah (peace be upon him) and his two Companions. She exposed three graves. They were neither high nor flat, and they were covered with gravel.” [Sunan Abî Dâwûd]
It is disliked for a grave to be raised higher than that. When the Prophet (peace be upon him) sent `Ali b. Abî Tâlib to Yemen, he commanded him: “Do not leave a statue without destroying it or a high grave without leveling it.” [Sahîh Muslim]
It is, however, permissible to identify the grave with a stone. When `Uthmân b. Maz`ûn died, the Prophet (peace be upon him) ordered someone to bring a stone. The man could not carry it, so the Prophet (peace be upon him) carried the stone and placed it at the head of the grave. He said: “This is so that I know my brother’s grave and can bury next to him those who may die from his family.”
It is not good to write any verses of the Qur'ân on the gravestone, since this could be a form of disrespect for the Qur’ân. Moreover, some ignorant people may see such a gravestone with verses on it and imagine that it is a distinguished grave that has special qualities or blessings associated with it.
And Allah knows best.
Source: Islam Today