You draw me, you hard-hearted adamant;
But yet you draw not iron, for my heart
Is true as steel: leave you your power to draw,
And I shall have no power to follow you.
The word "adamant" has always referred to extremely hard materials, typically diamond. In the Middle Ages, the word also referred to a magnet through a confused translation of the Latin "adamare," to love or be attached to. Shakespeare cleverly uses both meanings, describing Demetrius as both hard-hearted and a magnet.
A magnet will attract (draw) both iron and steel. Steel is stronger and easier to sharpen than iron. In addition, in Shakespeare's day, it was difficult and expensive to make steel. Helena says that Demetrius is not drawing common iron, but rather a quality steel. Her heart is true (faithful) and dependable as a device made of fine steel.