Agate, Heliotrope, Onyx, Plasma, Sard, are all varieties of Calcedony differently coloured by metallic oxides.
Native form: botryoidal (grape-like) masses ; but more frequently found in rolled pebbles.
Pure carbon. Sp. Gr. 3'55 (inferior to the Sapphire).
Hardness = 10, the highest in the scale. Highly electric by friction.
form : an octahedral crystal, usually modified by the obliteration of
the angles and edges : found mixed with gold-dust in a hard ferruginous