a large mass or block of hard stone rooted in the ground.

Rock-crystal, otherwise called sprig-crystal, in natural history, a name given to the third order of crystals, from their being affixed to a rock or other solid body.

This kind of crystal is the most common of all others, and is what the generality of authors describe under the name of crystal of the shops, being that kept for medicinal purposes.

The clearest, purest, and most transparent that can be had, ought to be chosen; and to prove its genuineness, it may be tried with aqua fortis, true crystal making no effervescence with that menstruum.