[dong (OP)]

I have two questions, could you please give me your opinion ?

Which ones are most useful in discriminating between minerals that look like the similar?
Describe a strategy that would allow you to determine that an unknown clear calcite crystal is indeed not the same as a known clear crystal of quartz?

[RD]

Calcite will fizz in weak acid , quartz will not.

If the quartz is pure it will be much harder than calcite, (i.e. calcite will be more easily scratched). http://en.wikipedia.org/wiki/Mohs_scale_of_mineral_hardness#Intermediate_hardness [Links inactive - To make links active and clickable, login or click here to register]

[blakestyger]

If you've got the kit, I imagine that refractive index should help separate some minerals.

[Bored chemist]

If you happen to have an X ray diffractometer...

[Bass]

The easiest way to distinguish quartz from calcite......

 [ Invalid Attachment ]

clacite has it, quartz doesn't

[Bored chemist]

I don't see how that picture shows "the ability to make 2 images of one object" (calcite has it, quartz doesn't).

http://en.wikipedia.org/wiki/Iceland_spar [Links inactive - To make links active and clickable, login or click here to register]

But I'm prepared to look at it until I do understand.
(Incidentally, from my point of view, I have a better chance of getting my hands on an X ray diffractometer.)

[Bass]

Cleavage- calcite has it, quartz doesn't

[frethack]

This is certainly an example of calcite because the cleavage is perfect.

[RD]

This is certainly an example of calcite because the cleavage is perfect.

I suspect a high silicone content  []

[Chemistry4me]

This is certainly an example of calcite because the cleavage is perfect.

I suspect a high silicone content  []

HAHA...LOL

[JimBob]

This is certainly an example of calcite because the cleavage is perfect.

No need to laugh, they are real. I sent Bass this picture of my girlfriend and he cropped it to emphasize the geological point to be made. (Notice the plural.)
 
I suspect a high silicone content  []

HAHA...LOL

[frethack]

This is your girlfriend, Jim? 

I know how you like to share!....hehehehhe

[techmind]

I have two questions, could you please give me your opinion ?

Which ones are most useful in discriminating between minerals that look like the similar?
Describe a strategy that would allow you to determine that an unknown clear calcite crystal is indeed not the same as a known clear crystal of quartz?

X-ray diffraction is the catch-all way to do crystallography (but the machine is likely to be at least as big as a washing-machine and cost a lot more).

Quartz is often found naturally growing as hexagonal-cross-section rods (not uncommon in some areas of Wales). I don't know what shape your unknown calcite might be.

Quartz is piezoelectric, so if you deposit some metal electrodes on it and apply a high voltage you can make it move fractionally (audibly click).

If you bash two lumps of quartz together in the dark you get a yellowy flash of light from them (and a strange smell). I've seen this with polycrystalline quartz, like the white pebbles you get on the beach.

Because of the piezo electricity, if you hit quartz with a hammer in the dark, you might get to see some sparks? Or just squeezed some in a vice, then released it again?

[JimBob]

This is your girlfriend, Jim? 

I know how you like to share!....hehehehhe

Forgive him.

He is still but a child - and will remain so all his life. He amuses so easily!