Albertus holds a transparent crystal up to the light in a monastery garden.

Rock crystal between water and air

Albert the Great (c. 1200–1280), a German Dominican scholar and investigator of nature, asks in his book on minerals what different stones can arise from. Rock crystal claims particular attention. It is firm enough to retain sharp edges while allowing the eye to penetrate its interior. Somebody attending only to its brightness might compare it to clear water. Somebody picking it up would feel a hard body held between their fingers.

The puzzle of a transparent body

For crystal appears to have as its matter not water alone, but a wateriness tending towards the nature of air. For this reason it is exceedingly transparent and tends towards brightness.

– Albertus Magnus, On Minerals, 1890, p. 15.[1]

Albert tries to describe both experiences within his account of the elements. He speaks of something watery and brings it close to what is airy. The careful wording leaves room for degrees. A reader is invited to imagine a passage between qualities generally noticed in quite different surroundings. Clear water transmits light and has a weight that can be felt when one lifts a vessel containing it. Air, in ordinary circumstances, offers even less resistance to the eye.

What a comparison does for a specimen

His choice of a word meaning “seems” is worth noticing. The scholar is moving from a feature observable in a crystal to a conjecture about its origin. An attentive reader can retrace the initial experience and still recognise the explanation as a historical one. To the hand this remains a stone with definite surfaces and boundaries. The eye can look right through the very same object. Any description trying to do justice to the singular impression made by rock crystal has to find room for both experiences at once.