| OBSIDIAN (Volcanic Silica Glass) | |
| OKENITE (Hydrated Calcium Silicate) | |
| OLIGOCLASE (Sodium Calcium Aluminum Silicate) | |
| * | OLIVINE (Group) |
| OLIVINE (Magnesium Iron Silicate) | |
| OLIVENITE (Copper Arsenate Hydroxide) | |
| 0 | ONYX (variety of Quartz - Silicon Oxide) |
| OPAL (Hydrated Silica) | |
| * | ORGANICS (Class) |
| ORPIMENT (Arsenic Sulfide) | |
| ORTHOCLASE (Potasium Aluminum Silicate) | |
| 0 | OSBORNITE (Titanium Nitride) |
| OSUMILITE (Hydrated Potassium Sodium Iron Magnesium Aluminum Silicate) | |
| 0 | OTAVITE (Cadmium Carbonate) |
| * | OXIDES (Class) |