calcedon

Any common mineral, such as chalcedony or calcite, that is found as an intrusion or vein in another stone, especially in a precious stone.

Merchandise invoiced as calcedon, classified at 60 per cent ad valorem under paragraph 1528, tariff act of 1930, is claimed to consist of imitation semiprecious stones, faceted, dutiable at 20 per cent under the same paragraph.

A yellowish-red carnelian is a calcedon. Is found–among other places–in India. […] Like carnelian, agate is a calcedon.

The bass lute, an instrument used by baroque composers (notably Telemann).

In this, the final chapter of the "Calc." saga, it can be stated that the that the abbreviation in question really does stand for calcedon, a theorbo-type instrument.

Indeed, the calcedon seems principally intended as a reinforcement to the cello: where the cello and keyboard parts differ from one another, the calcedon goes with the bowed instrument.