Above is a passage from Māhaprajñāparamitopadeśa. It mentioned that the Vajra can be broken by placing it on a tortoise shell and smashing it with goat's horn. Not sure if its literal meaning can be true from a physical science aspect. (I do not have a vajra, tortoise shell, neither goat's horn to experiment with)
If the above is true, Vajra should not be referring to diamond right? If so, what does Vajra means in this context?
I'm pretty sure that it does refer to "diamond" here.
I think - though am not 100% sure - that the term "金剛" as a translation was coined by Kumarajiva. However, in some other contexts, he uses "電". eg.
《成實論》卷12〈163 八解脫品〉：「問曰。行者若無禪定。云何能得身心空。及盡諸煩惱。答曰。是人有定而不能證。更有如電三昧。因是三昧得盡煩惱。如經中說。我見比丘欲取衣時有煩惱。取衣已即無煩惱。如是等所以者何。心如電三昧如金剛。真智能破煩惱。又此義佛第三力中說。所謂諸禪解脫三昧入垢淨差別如實知。於中禪名四禪。有人言。四禪四無色定皆名為禪。解脫名八解脫。三昧名一念中如電三昧。入名禪解脫三昧中得自在力。如舍利弗說。我於七覺中能自在出入。故知慧解脫阿羅漢有諸禪定。但不能入。深修習故能自在入。」(CBETA, T32, no. 1646, p. 339, c15-27)
Though this may seem strange to us now, because we've got used to the former, this is actually quite correct when we look at the older Indian meaning and sense of "vajra" / "vajira". ie. as Indra's thunderbolt weapon.
At first, commentators and translators seemed to alternate between the kind of lightning / thunder and diamond / adamantine explanations. Later, the former seems to win out.
Apparently goat horn and tortoise shell are also extremely hard.