Common or potch opal, 6cm x 4cm Locality unknown
Opalised ammonite, 6,5 cm x 4,5 cm Madagascar
 
Crystal system:                Amorphous                                
Hardness
:                        4,5 – 6,5
Density:                           1.9 – 2.3                                                    
Streak:                            White   
Cleavage:                         None                                          
Composition:  ...

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