Gibbsite
| Boehmite | Gibbsite | |
|---|---|---|
| Heft | normal for its size | light for its size |
Mineral
Boehmite
- Hardness (Mohs)
- 3–3.5
- Specific gravity
- 3–3.07
Properties
- Streak
- white
- Luster
- dull, earthy, pearly, or glassy
- Cleavage
- none or one direction
- Magnet
- not magnetic
- Crystal system
- orthorhombic
- Acid reaction
- no reaction
- Fluorescence
- does not glow
- Colors
- white, gray, yellow, brown, or red
Test it at home
The questions Stoneye asks, and the answer this mineral gives. A streak plate, a coin, a glass jar and a magnet cover most of them.
What colour is its streak?
white
How to do it
Drag an edge of the stone hard across the unglazed back of a bathroom tile or a porcelain plate. The powder line it leaves is the streak, and it is often nothing like the colour of the stone itself. A stone harder than the tile leaves no line at all, and that is an answer too.
Does the stone scratch your fingernail?
Yes
How to do it
Drag a sharp edge of the stone once across your thumbnail, pressing firmly, then wipe the nail. A groove you can feel with another nail means yes. A soft white mark that wipes off is powder from the stone, not a scratch.
Does the stone scratch a copper coin?
Either
Its hardness range crosses this reference, so either answer keeps it in the running.
How to do it
Lay a copper coin flat on the table and drag a sharp edge of the stone across it once. Wipe the coin afterwards: a line that survives the wipe is a scratch, a smear that comes off is powder. Use a plain copper coin, because plated coins are harder than the copper they look like.
Does the stone scratch glass?
No
How to do it
Lay a thick piece of glass, a jar or a bottle bottom, flat on the table and never hold it in your hand. Drag a sharp edge of the stone across it once, then wipe. A line you can feel with a fingernail is a scratch; a white smear that wipes away is powder from the stone.
Does the stone scratch a steel file?
No
How to do it
The easy way round is to run the file across a corner of the stone instead. If the file bites and leaves a groove, the stone is the softer one, so answer no. If the file skates over it and leaves only a grey metal smear that wipes off, the stone is the harder one, so answer yes.
Does the stone scratch quartz?
No
How to do it
Take a clear quartz crystal, or a piece you already know to be quartz, and lay it flat. Drag a sharp edge of the stone across one of its faces once, then wipe. Only a line that survives the wipe counts. If you have no quartz at hand, say you are not sure rather than guessing.
Does a magnet pull it?
not magnetic
How to do it
Hang a fridge magnet from a thread and bring it slowly towards the stone. Strong means the stone jumps to the magnet; slight means the hanging magnet only leans towards it. Wipe the stone first, because iron dust from a hammer or a file makes any stone look magnetic.
How does the surface shine?
dull, earthy, pearly, or glassy
How to do it
Turn the stone under a lamp or daylight and watch the reflection on a fresh, clean face. Metallic shines like polished steel and lets no light through its edge; glassy looks like the broken edge of a bottle. A dusty, weathered crust lies about the shine, so judge a broken face.
In how many directions does it break flat?
none or one direction
How to do it
Hold a broken corner in raking light and count the directions in which flat, mirror-like faces repeat. Faces parallel to each other count as one direction. A curved, shell-like break is fracture, not cleavage: if that is all you see, this stone has no cleavage.
How heavy is it for its size?
normal for its size
How to do it
Take an ordinary pebble of about the same size in your other hand and lift both. Heavy means clearly heavier than the plain stone; very heavy means it surprises you, the way a piece of lead does. Judge it against size, not against how big it looks.
Often confused with
Diaspore
| Boehmite | Diaspore | |
|---|---|---|
| Does the stone scratch glass? | No | Yes |
Where it is found
weathering crusts