Gaius Julius Solinus [3rd century CE] (De mirabilibus mundi / Polyhistor, Chapter 52.56-60): [Chapter 52.56] The first place among Indian stones is given to the diamond, inasmuch as it drives away madness, resists poisons, and expels empty fears of the mind. [Chapter 52.57] It is fitting to mention these things about the diamond first, as we consider them practical uses. Now we shall return to the appearance and colors of diamonds. An excellent sort is to be found in a certain type of crystal. Its unclouded radiance is similar to that of the substance in which it is formed. It is smoothed into a hexagonal point on both sides, so as to be cone-shaped, and it is never found above the bigness of a hazelnut kernel. [Chapter 52.58] Closest to this in distinction is the type found in gold. It is paler, and shines rather with the color of silver. The third kind occurs in veins of copper, and is nearer to the appearance of brass. The fourth is gathered from iron. It excels the others in weight, but not in strength. [Chapter 52.59] For both this kind and the kind found in copper can be broken, and for the most part can also be pierced by other diamonds. But the kind which we first mentioned are not conquered by iron, nor subdued by fire. Nevertheless, if they are soaked for a long time in goat’s blood, after several hammers and anvils are broken and wasted, they finally yield and split into shards. These fragments are in demand among engravers, who use them for inscribing gems. [Chapter 52.60] Between the diamond and the lode-stone there is a certain hidden dissension. If the diamond is placed near, it does not allow the lode-stone to snatch iron. Or if the lode-stone has already drawn iron to itself, the diamond snatches it away and steals it, as though it were booty. - [Arwen Apps translation, 2011]

Augustine [5th century CE] (City of God, Book 21, chapter 4): The diamond is a stone possessed by many among ourselves, especially by jewelers and lapidaries, and the stone is so hard that it can be wrought neither by iron nor fire, nor, they say, by anything at all except goats blood. Let me further say what I have read about the magnet. When a diamond is laid near it, it does not lift iron; or if it has already lifted it, as soon as the diamond approaches, it drops it. These stones come from India.

Isidore of Seville [7th century CE] (Etymologies, Book 16, 13:2-3): Adamas is a stone that is an unconquerable despiser of steel and of fire, yet it is softened by the fresh, warm blood of stags, and then is shattered by many blows of an iron instrument. It is said to reveal poisons as does amber [electron], to drive away useless fears, to resist evil arts.

Guillaume le Clerc [ca. 1210 CE] (Bestiaire, Chapter 39): Far away up in the East / There is a high mountain stretching far, / Where one finds a hard stone / When one looks for it on a dark night. / It does not shine at all by day / For then it loses its brightness. / The brilliance of the sun in truth / Robs it of its own strong light. / It is the diamond of which I tell you, / There is no stone in the world so hard. / No other stone can break it / Nor iron nor fire destroy it. / But they who wish to split it / Are used to break it with an iron hammer / When it has been steeped in he-goat’s blood. / In this way is it fractured. / But it is needful that the blood / Be quite fresh and not be cold. / With the pieces may one cut / Gems and iron and steel. / And moreover I teach you of the diamond / That the stone is not big at all. / Its colour is like to iron / And its lustre to crystal. / Some say of the diamond / That it is potent against poison, / And that it drives away vain fears, / So that the wiles of enchanters / Cannot enchant him / Who is wont to carry this stone. - [Druce translation]