This is a topic that comes up often- probably daily, at least- in the online geology community: identifying rocks from a photo only is very tricky, and to describe the final consensus as "tentative" may even be an overstatement. It's perhaps best described as a hypothesis: an educated guess. As I've said before, after handling tens of thousands of samples, known and unknown, and hundreds of different kinds of rocks with hundreds of different kinds of textures and other features, the process of identification becomes internalized to the point that it's almost unconscious. Most important, it is by no means only visual. We use all of our senses in identifying a sample. As an analogy, imagine we saw the entire visual spectrum in a gray-scale, and someone sent you a photograph of a rainbow, asking for an ID. You might very well guess it's a rainbow, but there are quite a number of other phenomena that create a semi-circle of brighter light in the sky- all sorts of arcs and halos from ice crystal diffraction, for example. So a practiced cloud observer might well guess "rainbow" is the most likely identification, but without more information (position of sun relative to unknown, sky/weather conditions, for example), you're just guessing which is most likely, with no real way to be confident.
So I started a hashtag, #WebRockIDTips, which will be quite short-lived, I'm sure, to post some comments on features and characteristics that I and others would find helpful in pinning down a somewhat more confident identification of a geological photograph- whether it's a single rock or a whole outcrop. This post is to archive those comments for future reference. Tweets with no identifier are mine; others' tweets will include an identifier. Here's what we have so far, and I'll add others as they come in.
- Just got recommended as a geotweep to help ID rocks over twitter. What, along with photo, also helpful? Scale (ruler great!)
- Different photos with different angles, perspectives.
- Location, Location, Location- the closer to original source, the better- this can seriously narrow possibilities.
- Oddities- what is unusual about this rock? What brought it to your attention? Density? Luster? Grains? Why interesting?
- What do other geotweeps wish folks requesting rock photo ID's would include as helpful info?
@maitri Pantone/web color and texture.
@meaganhogg Hardness. Anyone can grab a knife and scratch. Have you asked http://www.reddit.com/r/geology ? Lots of Rock ID Qs there.
- Water-rounded surfaces almost useless by themselves, but in conjunction with freshly-broken surface, can add info.
- What, if any, minerals or features can you identify with some degree of confidence?
@rschott My foremost recommendation is a plea for photos that are in focus (admittedly not always easy at the macro scale).
@rschott Beyond that I'd largely echo what you've suggested. Also advise the submittor to be prepared for non-definitive answers. @rschott Probably my best advice is to post the mystery rock photos to a blog that allows comments, and point to it from Twitter.
@rschott Best to get a conversation going around the sample on a space the poster can call their own.