I found this new site that will help you do calculations on jewelry and on coins that have various weights and purity. Gold comes in various purities such as 10 karat 14 karat etc. These are expressed as 10K 14K 18K 22K 24 K.

Pureness in karats is recorded in 24ths. In other word 14k gold is 14 parts gold out of 24 parts. 24 parts being PURE.
For Example: I have a ring that is 34.52 grams in total weight. It is 14k pure. That is 14/24 parts pure. Which is 58.3% Gold, The weight of the total Actual Gold Weight is .647 of and troy ounce. Based upon 810USD per ounce there is 524 FRN dollars worth of Gold

Weight of Alloy	34.52 Grams - 22.20 Dwts - 1.110 Troy Ozs
Alloy	Gold of 0.583 Purity
Weight of Metal	0.647 Troy Oz of Pure Gold
Price Used	Using the specified 810.00 USD / Troy Oz
Value of Metal	524.45 USD

The calculator on this page will help you determine the value and the weights based on several karat calculations. Calculator

Here is some more information on weights that will help you understand values and conversions

1 troy ounce	= 480 grains = 20 pennyweights = 1.097 avoirdupois ounces = 31.103 grams
1 troy pound	= 12 troy ounces = 5760 grains = 240 pennyweights = 13.164 avoirdupois ounces = 0.82275 avoirdupois pounds = 373.236 grams

And for even more detail:

American Marking (karatage)	Pure Gold content (Fineness in percent)	European Marking
24 K	100	1,000
22 K	91.6	916
20 K	83	833
19 K (used in Portugal)	79.2	792
18 K	75.0	750
15 K (seen in antiques)	62.5	750
14 K	58.3	585
12 K	50.0	500
10 K	41.7	417
9 K	37.5	375

And for even more information and charts: From Kitco:

Conversion Table

Pennyweight to Ounces to Gramss

DWT OUNCE GRAM DWT OUNCE GRAM 10 0.5 15.550 340 17 528.760 20 1 31.104 350 17.5 544.311 30 1.5 46.655 360 18 559.863 40 2 62.207 370 18.5 575.415 50 2.5 77.759 380 19 590.967 60 3 93.311 390 19.5 606.518 70 3.5 108.862 400 20 622.070 80 4 124.414 410 20.5 637.623 90 4.5 139.966 420 21 653.174 100 5 155.518 430 21.5 668.725 110 5.5 171.069 440 22 684.277 120 6 186.621 450 22.5 699.829 130 6.5 202.173 460 23 715.381 140 7 217.725 470 23.5 730.932 150 7.5 233.276 480 24 746.484 160 8 248.828 490 24.5 762.036 170 8.5 264.380 500 25 777.588 180 9 279.932 510 25.5 793.139 190 9.5 295.483 520 26 808.691 200 10 311.035 530 26.5 824.243 210 10.5 326.587 540 27 839.795 220 11 342.139 550 27.5 855.346 230 11.5 357.690 560 28 870.898 240 12 373.242 570 28.5 886.450 250 12.5 388.794 580 29 902.002 260 13 404.346 590 29.5 917.553 270 13.5 419.897 600 30 933.105 280 14 435.449 610 30.5 948.657 290 14.5 451.001 620 31 964.209 300 15 466.553 630 31.5 979.760 310 15.5 482.104 640 32 995.312 320 16 497.656 650 32.5 1010.864 330 16.5 513.208 660 33 1026.300