Lieutenant Colonel insignia is identified by a Silver Maple Leaf. In the US Army it is a cluster of silver OAK leaves. In the British army it is a crown and a single 'pip'.
Subdued rank insignia in the US Armed Forces wasn't introduced until the Vietnam war.
Which insignia did you have in mind, since there's a few different insignia worn? There's rank insignia, branch insignia, and regimental crests.
On 18 June 1780.
Insignia of rank
In the US services Generals wear stars. Majors and Lt. Colonels wear an Oak Leaf; the gold oak leaf is the Major's insignia, and the silver oak leaf is the Lieutenant Colonel's rank insignia. Major is the first rank entering the FIELD GRADE levels, that is why it is gold. Other countries have different insignia to denote rank.
Insignia of rank
Very basically speaking, the difference is one step in rank. While a Lt. Colonel is normally referred to a Colonel, a Colonel is sometimes to referred to as a "full bird colonel" to distinguish the difference. In the Army at least, a Lt. Colonel wears a clover leaf rank insignia (silver, I think) but a full Colonel wears an eagle insignia, hence the reference "full bird."
Yes, they do, except for the Army Combat Uniform, which does not have epaulets.
bjmp ranks
For the US Army, it's a bronze oak leaf. The same insignia is used by Majors in the Air Force and Marine Corps, and by those holding the rank of Lieutenant Commander in the Navy and Coast Guard while wearing certain uniforms.
The word insignia means emblem of military rank, a sign or a token and it comes from the Latin language: IN - towards, SIGNUM - sign. Later it transforms to insignis and then insigne and around the 17th century the word insignia is finalized. Plural can be insignia, as well as insignias.