I have a small board powered by a button cell battery. The voltage range for the microcontroller on the board is 1.9V to 5.5V, while a single battery provides 3V. This leads to quick battery depletion, so I'm considering using two button cell batteries with a total voltage of 6V and adding a diode for regulation. The diode I'm using is the one below. Is this diode made of silicon or germanium? Also, what is its voltage drop, and I'm unsure about its reliability with this setup.
There is nothing below, what is the diode's part number?
It is the mAh of the battery that determines how long it lasts with a given load. The diode and higher voltage will only keep the battery voltage up higher. Connecting the batteries in parallel will make them last about twice as long. Replacing the button cells with 3 AAA or AA batteries will run it much longer and probably save money on battery costs.
Do you have a multi meter with a diode tester?
If it shows near 0.6V it is a silicon diode.
Less could be germanium (but those are not very common) or schotky. Other types are also possible (zener diodes). So best is to start from the part number.
Button cells are pretty expensive with regard to mAh stored...
I moved your topic to an appropriate forum category @topfastpcb.
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Thanks in advance for your cooperation.
Hi, @topfastpcb
Welcome to the forum.
What micro-controller are you using?
Can you please post a copy of your circuit, a picture of a hand drawn circuit in jpg, png?
Hand drawn and photographed is perfectly acceptable.
Please include ALL hardware, power supplies, component names and pin labels.
Thanks.. Tom.. 
A diode is not a regulator.
Also, what is its voltage drop, and I'm unsure about its reliability with this setup.
Definitely not reliable. The voltage drop could be anywhere from a few tenths of a volt to 1 volt or more depending on the current. You may exceed the 5.5V maximum.
A typical voltage V current graph for a PN junction. Note the Y axis is in Amps.
For small currents in the mA region a diode voltage drop is considered to be 0.7V.
Can we just address the elephant in the room and ask why you're using a button cell instead of something more appropriate?
Your idea should work.
The diode wastes a little energy but with two batteries you're starting-out with twice the energy and you can drain the batteries down to 1.75V each (or average) and still have 1.9V for your circuit (if I did my math correctly),
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