Siderite with Hyalite Opal
Siderite (Iron Carbonate) with inclusions of Opal-AN (Hyalite) · Mineral Specimen / Collector Mineral
Crystal System
Trigonal (Siderite) / Amorphous (Opal). Habit: Rhombohedral, massive, or botryoidal crusts.
Mineral Group
Carbonates (Siderite) and Oxides/Silica Group (Opal)
Chemical Formula
FeCO3 with SiO2·nH2O
Formation Process
Hydrothermal or sedimentary precipitation. Hyalite often forms as secondary botryoidal coatings in volcanic cavities or hydrothermal veins.
About This Crystal
Massive brown to yellowish-brown siderite matrix featuring a distinct 'bubble-like' or botryoidal coating of hyalite opal. Texture is grainy to crusty with a vitreous to resinous luster.
Physical Characteristics
Color: Brown, tan, and clear; Streak: White; Cleavage: Perfect rhombohedral (Siderite); Fracture: Conchoidal; Specific Gravity: 3.8 (Siderite) / 2.1 (Opal); Fluorescence: Hyalite often shows strong green fluorescence under UV.
Optical Properties
Siderite is uniaxial negative with high birefringence; Hyalite is isotropic (singly refractive) and typically colorless/transparent.
Hardness & Durability
Origin Region
Notable sources include Hungary, Mexico (Zacatecas), and certain alpine veins in Europe.
Hardness & Durability
Siderite: 3.5-4.5; Hyalite Opal: 5.5. Brittle tenacity; not suitable for most jewelry due to low hardness and cleavage issues.
Care & Maintenance
Avoid acids (reacts with siderite) and ultrasonic cleaners. Keep away from extreme heat to prevent dehydration of the opal component.
Rarity & Value
Specimen quality is uncommon; value depends on the clarity of the hyalite and the presence of UV fluorescence. Generally affordable for collectors.
Special Characteristics
The globular (botryoidal) 'glass-like' bubbles on the surface are characteristic of Hyalite (Opal-AN), which often glows neon green under short-wave UV light.
Lore & History
Siderite was historically an iron ore. In metaphysical lore, it is believed to help with alignment and grounding; Hyalite is associated with emotional clarity and focus.