From The Gemology Project
Revision as of 11:59, 22 March 2006 by Doos (talk | contribs) (Cosine)
Jump to: navigation, search

Although the study of gemology requires no formal prior training, a highschool diploma would make it easier to understand basic math. Especially knowledge of trigonometry might serve you well.

Below are some basic calculations that you may want to understand.

Sine, cosinus and tangent


The sine, cosine and tangent are used to calculate angles.

In fig.1 the 3 sides of a right triangle (seen from corner A) are labeled Adjacent side, Opposite side and Hypothenuse. The hypothenuse is always the slanted (and largest side) in a right triangle.

The opposite and adjacent sides are relative to corner A. If A would be at the other accute corner, they would be reversed.



Sine is usually abbreviated as sin.

You can calculate the sinus of a corner in a right triangle by dividing the opposite side by the hypothenuse. For this you need to know two values:

1. the value of the opposite side
2. the value of the hypothenuse.

In Fig. 2 those values are 3 and 5, the sinus of A or better sin(A) is 3/5 = 0.6

<math>\sin = \frac{opposite\ side}{hypothenuse} = \frac{3}{5} = 0.6</math>

Now that you have the sin of corner A, you would like to know the angle of that corner.
The angle of corner A is the "inverse sine" (denoted as sin-1 or arcsin) of the sinus and is done by complex calculation. Luckely we have electronic calculators to do the dirty work for us:

  • type in 0.6
  • press the "INV" button
  • press the "sin" button

This should give you approximately 36.87, so the angle of corner A is 36.87°

<math>\arcsin \left(\sin A\right) = \arcsin \left(0.6\right) = 36.87</math>


The cosine of a corner in a right triangle is similar to the sine, yet now calculation is done with the division of the adjacent side by the hypothenuse. The cosine is abbreviated as "cos"

In Fig. 2 that would be 4 divided by 5 = 0.8

<math>\cos = \frac{adjacent\ side}{hypothenuse} = \frac{4}{5} = 0.8</math>

Again as with the sine, the inverse of the cosine is the arccos or cos-1:

  • type in 0.8
  • press INV
  • press cos

This should give you 36.87 aswell, so the angle remains 36.87° (as expected).

<math>\arccos \left(\cos A\right) = \arccos \left(0.8\right) = 36.87</math>


The 3rd way to calculate and angle is through the tangent (or shortend "tan"). The Tangent of an angle is opposite side divided by adjacent side.

<math>\tan = \frac{opposite\ side}{adjacent\ side}</math>

For Fig.2 that will be 3/4 = 0.75
Calculation of the angle is as above, but using the arctan or tan-1:

  • type in 0.75
  • press INV
  • press tan

This should give you 36.87, so through this method of calculation the angle of corner A is again 36.87°.

<math>\arctan \left(\tan A\right) = \arctan \left(0.75\right) = 36.87</math>

A simple bridge to remember which sides you need in the calculatios is the bridge SOH-CAH-TOA.

  • SOH = Sine-Opposite-Hypothenuse
  • CAH = Cosine-Adjacent-Hypothenuse
  • TOA = Tangent-Opposite-Adjacent

Degrees, minutes and seconds

When we think of degrees we usually associate it with temperature and we consider minutes and seconds as attributes of time. However in trigonometry they are used to describe angles of a circle (we name them the radian values).

A full circle has 360 degrees, or 360°.
Every degree can be divided into 60 minutes (like in a clock) instead of the 10 decimal subdivisions.
Minutes are notated with a ', as in 26'.
The individual minutes are further divided into 60 seconds and they are described with '', as in 23''.

This may look odd at first, but it's not very hard to understand.

If you have an angle of 24°26'23'' (24 degrees, 26 minutes and 23 seconds), this means that the decimal value is:

  • 24°
  • 26 divided by 60 or 26/60 = 0.433°
  • 23/(60 * 60) or 23/360 = 0.063°

This totals in 24 + 0.433 + 0.063 = 24.439° in the decimal value (which is the decimal value of the critical angle of Diamond).

When you want to calculate the radian value of 24.439°, you do the following:

  • the 24 stays 24 (because that doesn't change)
  • you try to find how many times 0.439 times 60 fits in the degree by: 60 times 0.439 = 26.34, so that is 26 full minutes (0.34 left over)
  • you calculate the seconds through 60 times 0.34 = 20.4 (or 20 full seconds because we don't count lower than seconds).

This gives 24°26'20'' (24° + 26' + 20'') instead of the 24°26'23''. The 3 second difference is caused by the rounding down to 3 decimals in the prior calculation. In gemology we usually don't even mention the seconds, so it will be rounded down to ≈ 24°26'.

Even though you may not need this knowledge often, it is important that you atleast know of its excistance as you may get confused when reading articles. Sometimes values are given in decimal degrees, at other times in radian values.