You have the 6c Lincoln from the years 1870-1882. It is fairly common To value the stamp you have to first look for a what is called a 'Secret Mark' and look for an embossed pattern, called a grill, on the reverse of the stamp. An example is shown below.

If the stamp has a grill, then click on the numbers 134-144 on the menu to the left.

If the stamp has no grill then examine the bottom left corner of the stamp.

On the bottom left corner the four left most lines of the ribbon have thicker shading. Below this text there are two images, one showing the thicker shading and one with normal shading.

If you see the thicker shading in the ribbons, as shown below then click on the numbers 182-191 on the menu to the left. If the stamp has a date cancel and you can see the date is prior to February 3rd, 1879 then click on the numbers 156-166 on the menu to the left. All unused stamps belong to the latter period of numbers 182-191.

If you do not see the thicker shading in the ribbon (shown below) then click on the numbers 145-155 on the menu to the left.

Lastly look at the lines that border the image. If there are only three lines, as shown below, they you have the 1882 printing by the American Bank Note Company. The value of these can be found by clicking the numbers 205-209 on the menu to the left.

Showing thicker shading inside ribbon

Normal shading inside ribbon

Three border lines

Four border lines

Example of a grill

This stamp was reprinted in 1875 and it can be viewed by clicking on the numbers 167-177. These are all accounted for, so you will not own this.