RocksdayBlastoid vs Crinoid
To tell Blastoid from Crinoid: Crinoids are usually found as stacked stem discs, not a five-sided bud.
An extinct relative of crinoids and starfish whose body fossil looks like a small flower bud with five grooves.
Photo: Diego Delso, CC BY-SA 4.0
Fossil sea lilies — animals related to starfish — usually found as stacked, button-shaped stem pieces.
Photo: James St. John, CC BY 2.0
Key differences
- Crinoids are usually found as stacked stem discs, not a five-sided bud.
Side by side
| Property | Blastoid | Crinoid |
|---|---|---|
| Type | Fossil | Fossil |
| Mohs hardness | 3 | 3 |
| Luster | Dull | Dull |
| Colors | Gray, White, Brown | Gray, White, Brown |
| Crystal system / structure | Calcite bud with five ambulacra | Calcite stem discs |
| Specific gravity | 2.6–2.8 | 2.6–2.8 |
| Found in | Burlington Limestone, Iowa; Illinois | Burlington Limestone, Iowa; Indiana |
Properties that are the same for both are grayed out.
How to identify blastoid
- A nut- or bud-shaped calcite fossil.
- Five food grooves running from the top toward the base.
- Fizzes in acid. No stem discs unless a crinoid is in the same rock.
Structure: Calcite bud with five ambulacra
Full Blastoid guide →How to identify crinoid
- Small round discs, often with a hole in the center.
- Stacked into stem segments like beads.
- Fizzes in acid (calcite).
Structure: Calcite stem discs
Full Crinoid guide →More comparisons
Frequently asked questions
What is the difference between blastoid and crinoid?
Blastoid is a fossil with a Mohs hardness of about 3 and a dull luster, usually gray, white or brown. Crinoid is a fossil with a Mohs hardness of about 3 and a dull luster, usually gray, white or brown.
How can you tell blastoid from crinoid?
Crinoids are usually found as stacked stem discs, not a five-sided bud.
Could you tell them apart from a blurry close-up?
Play today's Rocksday