I bought a necklace in China that I was told was made of agate.  Back
in the US I was told it was carnelian, a relatively cheap agate.  But
I'm wondering if the beads may not even be carnelian, but glass. 
They're guite uniform looking, not what you'd expect to find in
nature, but I've read that agate can dyed.  How can I tell for sure if
they're glass or real?

Hello there

We can cover a couple of possibilities.

Carnelian is the clear red to brownish red member of the Chalcedony
family. It is a microcrystalline variety of Quartz (Silicone Dioxide)
and may contain small amounts of iron oxides.  Some carnelian,
however, is brown, yellow or even white.  The hardness is 7, and the
streak is white. (glass is about hardness 5 )

It may be (probably) that you do have true carnelian but carnelian
which has been treated to produce uniformity of color.  Heat treatment
is not a fraudulent practice when used on certain gems, if the results
are permanent.  Carnelian is one of those gems.  You can find out more
here:
http://www.gemshopper.com/fraud.html - "Fraud risk - gemstones"

Even though carnelian is found mostly in Greece and Asia Minor, China
does have its own native source and processes a lot of the stone.  In
fact, carnelian has been used in china for centuries.  "Buddhists in
China and India created amulets inlaid with Carnelian and other
semi-precious stones, ascribing to them powers of protection and
utilizing them for many rituals." - Quote from "References to
Carnelian in Ancient and Modern Myths and Legends"
http://www.jewelrysupplier.com/2_carnelian/carnelian_mythology.htm - -
you will also find many links here to additional information about
carnelian.

http://www.jewelrysupplier.com/2_carnelian/carnelian_properties.htm -
for the physical properties of carnelian

As you can see in these photos, uniformity in appearance is not all that unusual.
http://www.bkexports.com/Carnelian.htm

Since chalcedony is one of the harder stones, try to scratch one of
your beads with a small piece of glass.  If you have true carnelian or
even simply 'treated' chalcedony, the glass will not scratch the bead.
 Also, if one of your carnelian gems has a sharp edge, use it to try
and scratch a piece of glass.  The carnelian will leave a scratch on
the glass.

One other thing to consider is that chalcedony gem stones are so
common and found in so many colors that making glass paste imitations
of them would hardly be cost effective thus creating little desire to
duplicate the substance in glass.  The simple scratch test as
mentioned above should put your mind at rest.

search - Google

terms - carnelian, chalcedony, carnelian agate, hardness testing, hardness scale

If I may clarify anything, please ask.

Cheers
digsalot