There are several types of gypsum, but the one which can be used for decorative purposes is called compatto. It is also known under the names of alabastrite and alabastro gessoso. It has as a base calcium sulphate, and it does not effervesce in acids. There are entirely opaque gypsums, others are translucent, and some entirely transparent. They are so soft that they can easily be marked with the fingernail, and therefore they either do not take a beautiful polish, or they soon lose it. Brongniart. 84
544. (135.1) Gesso della Castellina in Tuscany. Entirely of dirty snow white, and opaque. (Not common).
545. (135.2) Gesso di Volterra. Entirely white and transparent. Some people have thought that this may have been the Marmor Phengite of the ancients. It is used a great deal for vases and lamps, on account of its transparency. (Common).
546. (135.3) Gesso di Jesi. Mixture of white, and grey, transparent. (Not common).
547. (135.4) Gesso di Matelica. Grey ground streaked with darker grey. (Not common).
548. (135.5) Another from the same place. Light grey ground waved with white. (Not common).
549. (135.6) Another from the same place. Entirely light grey with some white lateral veins. (Not common).
550. (135.7) Another from the same place. Entirely dark grey delicately flowered with whitish. (Not common).
552. (136.9) Gesso di Faenza. Mixture of white and grey. Takes a poor polish. (Common).