Mineral Field Tests – or Tests on the Go

© Bert Ellison 1999 - 2002

Trouble identifying that precious piece that you tapped out of the quarry or dump? Don’t despair, if you’ll settle for a rough field estimate, but you’ll have to resort to more sophisticated tests if you want to be precise. So these are field tests only:

HABIT – is it flat and scaly like mica or in a crystal form? Crystals of unusual size and shape are rare – they are in museums! Since there are some thirty variations of the crystal systems, few of us are qualified to judge. A poor field use, though some forms are useful e.g. quartz.

COLOUR – useful in some cases but not reliable. Beware of oxidation or tarnish which hides the true colour. Also many minerals come in many hues e.g. quartz, calcite.

LUSTRE – the way light is reflected from a mineral. Of very little field use.

OPACITY – or TRANSPARENCY. Metallic minerals are opaque. Some transparent ones may be potential gemstones. Beyond sorting out the metallics, this property is of limited use but don’t toss away any emerald, topaz, ruby or sapphire!

SPECIFIC GRAVITY – or SG is of some use. Most metallics run about SG 3 to 4. "Stony" minerals are about 2 to 3. If it comes in over 6, stake a claim! Most dumps don’t offer specimens large enough to "heft" for us to judge. Use at least a good "thumbnail" size.

STREAK– press a piece across unglazed tile & note colour of powder. Very useful! Cuts through tarnish.

HARDNESS – or H. Get to know Mohs scale! This is a very useful quality and usually the first test one makes. Keep that knife handy! While many minerals may be similar, this test is great for sorting out the two great stoney groups – calcite/limestone etc. and quartz. Hardness alone may at least put you on the right track. Good for metallics too – try pyrite vs. gold (6.5 vs. 2.5). Excellent first test but some minerals are harder in certain directions. E.g. kyanite; 4-5 lengthwise, 6-7 across the crystal.

CLEAVAGE – not the burlesque type but the way a piece breaks. Shell-like (conchoidal) yields sharp shards (as flint). Some yield smooth flat breaks (as micas) and some are partly smooth & rough in different directions (as feldspars). The quartz group – chert, flint, amethyst etc. – have very rough breaks. So do garnets. Of modest use but good for feldspars, quartz, micas, calcite, galena and halite for examples.

ACID – use 10% HCl (hydrochloric or "plumbers" acid) in squeeze bottle. Excellent to verify the carbonates from almost anything else, especially the quartz family. Great for limestone vs. dolomite. Fizzes slowly on cold rock. Warm it up first.

Oddballs – TASTE – don’t lick everything – there is lots of arsenic around! Great for halite and potash salts if you suspect them.

Oddballs – MAGNETIC – very useful for picking out magnetite, ground-up pyrrhotite (an iron sulfide). Use a horseshoe magnet suspended on a string.

Oddballs – FLUORESCENCE – of some use (in the dark) for fluorite, some calcite, scheelite and sphalerite .. Oh yes! And diamonds too!

Identifying Rocks & Minerals

1 DIFFERENCE between Rocks & Minerals