Meaning, he threw a stone of equal hardness and difficulty. The stone is used metaphorically for a stone because stones vary in their impact depending on the target; the smaller stones are thrown at the smaller ones, and the larger ones at the larger ones.
In the account of the Battle of Siffin, when Mu'awiyah sent Amr ibn al-As as an arbitrator with Abu Musa, al-Ahnaf ibn Qays came to Ali (may God honor his face) and said, "You have thrown a stone of unparalleled strength. Include Ibn Abbas with him, for he never tightens a knot without untying it." Ali wanted to do this, but the Yemenis insisted that one of the arbitrators be from among them. So, he sent Abu Musa. The meaning is: You have thrown a stone without equal; he is the stone of unparalleled strength, just as you might say, "So-and-so is the man of his time," meaning he has no equal among men.
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The closest equivalent in English to this meaning is:
He sent his best man.
That is, he dispatched his strongest and most capable men.
A very similar expression is:
He brought out the big guns.
That is, he used his strongest resources to confront the situation, which is very close in meaning to "throw a stone," i.e., a very strong and difficult weapon.
Similarly:
He met fire with fire.
That is, he met force with force and equal with equal.
For the meaning that implies "unrivaled," as in "a stone of the earth," one can say:
He is in a league of his own.
That is, a man without peer in his field.
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أي بقرنه الذي هو مثله في الصلابة والصعوبة، جعل الحجر مثلا للقرن لأن الحجر يختلف باختلاف المرمى، فصغار هذا لصغار ذاك وكباره لكباره.
وفي حديث صفين أن معاوية لما بعث عمرو بن العاص حكما مع أبي موسى جاء الأحنف بن قيس إلى على كرم الله وجهه فقال: إنك قد رميت بحجر الأرض، فاجعل معه ابن عباس، فإنه لا يشد عقدة إلا حلها، فأراد علي أن يفعل ذلك فأبت اليمانية إلا أن يكون أحد الحكمين منهم، فعند ذلك بعث أبا موسى، ومعناه: إنك رميت بحجر لا نظير له فهو حجر الأرض في انفراده، كما تقول: فلان رجل الدهر، أي لا نظير له في الرجال.
