Glass

Glass is an amorphous, non-crystalline solid substance that is not classified as a mineral due to its lack of an ordered internal structure.

## Characteristics Glass, from a mineralogical perspective, is not a mineral but an amorphous (non-crystalline) solid. This means that its atoms do not form an ordered, repeating crystal lattice, which is characteristic of minerals. Natural glass, called lechatelierite, is formed by the melting of quartz sand by lightning strikes (fulgurites) or meteorite impacts. However, it is very rare. In collecting, one mainly encounters volcanic glasses, such as obsidian, or anthropogenic (man-made) materials, which are sometimes mistaken for minerals. ## Physical Properties As an amorphous material, glass does not exhibit cleavage, and its characteristic feature is a conchoidal fracture with sharp edges. Hardness typically ranges from 5-6 on the Mohs scale, and density is approximately 2.2-2.6 g/cm³, depending on the composition. Luster is typically vitreous. ## Colors and Varieties Natural glasses, such as obsidian, are most often black, although they can also be brown, gray, or greenish. The coloration depends on the presence of impurities, e.g., iron compounds. Lechatelierite is usually colorless, whitish, or gray. Man-made glasses can have virtually any color due to added pigments.

Properties

Mohs hardness: 5-6.5
Luster: Vitreous
Streak: White
Density: 0
Fracture: Conchoidal
Transparency: Transparent,Translucent,Opaque

Diagnostic features

## Identification The key diagnostic feature of glass is its fracture – always perfectly conchoidal, with concentric, arcuate lines and very sharp edges. The absence of any flat parting surfaces (cleavage) and vitreous luster are also characteristic. In the case of obsidian, its low density compared to similarly looking minerals is helpful. ## Distinguishing from Similar Materials Glass is sometimes confused with quartz (especially morion or smoky quartz). However, quartz is harder (scratches glass), has poorly visible cleavage, and rarely exhibits such a perfect conchoidal fracture over its entire surface. From onyx (a variety of chalcedony), it is distinguished by its structure – onyx has a microcrystalline structure and often exhibits banding.

Geological environment

## Genesis Natural glass forms under conditions of extremely high temperature and rapid cooling, which prevents crystallization. The main environments of formation are: - **Volcanism:** Rapid cooling of acidic, silica-rich lava leads to the formation of obsidian. - **Impacts:** Meteorite impacts melt rocks, creating impact glasses (e.g., tektites, desert glass). - **Atmospheric Discharges:** Lightning strikes on quartz sand create tubular fulgurites, composed of lechatelierite.

Rarity

Common

For collectors

## Quality Criteria Since glass is not a mineral, its collector's value primarily applies to natural forms, such as obsidian, tektites, or fulgurites. For obsidian, uniform, deep black color, interesting inclusions (e.g., cristobalite spherulites forming "snowflake obsidian"), or iridescence ("rainbow obsidian") are valued. For tektites and fulgurites, well-preserved, characteristic shapes and specimen size are crucial.

Care and storage

## Cleaning Specimens of natural glass (e.g., obsidian) can be safely cleaned under running water with a soft brush and neutral soap. They should be thoroughly rinsed to avoid leaving residue. ## What to Avoid Glass is sensitive to rapid temperature changes (thermal shock), which can cause cracking. Contact with strong chemicals, especially hydrofluoric acid, which etches it, should be avoided. Despite its relatively high hardness, it is brittle and susceptible to scratching by harder minerals (e.g., quartz). ## Storage Glass specimens are best stored separately, in soft boxes or wrapped in fabric, to protect them from scratches and impacts. Avoid places exposed to drops or vibrations.