Starburst Agate Identification Guide

What Starburst Agate Looks Like

Starburst agate is a banded variety of agate (microcrystalline/chalcedony quartz, SiO2) whose internal pattern radiates outward like a burst or star, often from sagenitic needle inclusions or radial fibrous growth. Colors vary widely (white, gray, blue, red, orange, brown) and the radiating "rays" are typically a contrasting tone against banded or translucent ground. Luster is waxy to vitreous, and the stone is translucent on thin edges. As with all agate, look for curved, concentric banding combined with the distinctive star-like radial sprays.

Step-by-Step Field ID

1. Hold it to light. Agate is translucent at the edges; light passes through thin sections, distinguishing it from opaque jasper.
2. Look for banding plus radial rays. Concentric or fortification banding combined with needle-like sprays radiating from a point is the signature.
3. Test hardness. It scratches glass and steel (Mohs 6.5-7).
4. Check the surface luster. Polished pieces show a glassy to waxy shine; broken surfaces are conchoidal and smooth.
5. Examine for inclusions. The "starburst" often comes from sagenite (mineral needles) trapped in chalcedony.

Key Diagnostic Tests

• Mohs hardness: 6.5-7, scratches glass.
• Streak: white.
• Fracture: conchoidal, no cleavage.
• Specific gravity: ~2.60.
• Acid: no reaction (silica, unlike carbonate look-alikes which fizz).
• Translucency: translucent edges separate it from jasper.

Common Look-Alikes

• Jasper (including star-pattern jaspers): opaque even on thin edges and usually duller; agate transmits light.
• Glass / manufactured "starburst" cabochons: may show gas bubbles and swirl marks; lacks true mineral needle inclusions and natural fortification banding.
• Thunder egg / geode agate: related material; starburst refers specifically to the radial spray pattern within agate.
• Sagenitic agate generally: starburst is a subset where needles radiate from a central point rather than scattering randomly.

Where It Is Found

Agate forms in gas cavities (vesicles) of volcanic rocks and in some sedimentary settings worldwide. Radial/starburst material comes from agate fields such as those in Mexico, Brazil, the western United States (Oregon, Montana, Arizona), and India.