Looking up some information about mourning jewelry (often made of the previously-mentioned jet), apparently pearls are used to signify that the deceased was a child, which seems to play strongly into the Last Waste's theme of despair and loss. White enamel is also apparently used in mourning jewelry to indicate the deceased was a woman who died unmarried and a virgin; while perhaps not as useful for the rituals, I could see Sister María Teresa having a rosary of jet with white enamel accents in remembrance of her friend and fellow nun who's loss pushed her over the edge (she might also be able to use it as something of a magical focus).
As for how precious stones might be used, I'd suggest making use of Decanic trappings. You don't have to use the Decans for this, just the system. That only breaks things into "expensive" and "inexpensive," with the former just being anything worth half a month's income or more (I think it's specifically
that character's income, but making it match Average income for the campaign setting would probably be ok). Given that precious stones are going to be well beyond this, expanding it might be a good idea. TL8 Average monthly income is $1300, but we'll round this up to $1400 (so that half is $700). I'd be tempted to use something like the following:
Code:
$ DP
7 1
30 2
150 5
700 10
3k 20
15k 50
70k 100
300k 200
1.5M 500
7M 1000
30M 2000
150M 5000
700M 10000
This is for items consumed in the ritual; those that are simply used as foci, and thus are reusable, are treated as though they were worth 1 category less (if Sister María Teresa's mourning rosary is worth $3,000, that's 20 DP if she sacrifices it for a ritual, but only 10 if she uses it as a focus). This also assumes the object is an appropriate sacrifice; an
extremely appropriate sacrifice might be worth +1 category (I'd be tempted to do something like +1/2 category, 100 DP becomes 150 DP, say), while an only marginally appropriate one might be worth -1 category. Using more than one expensive component treats the second as being worth one category lower, the third two categories lower, etc - if you had 3 gemstones worth $15,000 each, and you consumed all three in a single ritual, the first would be worth 50 DP, the second 20 DP, and the third 10 DP, for a total of 80 DP. The above monetary values are minimums - a $100,000 gemstone is worth 100 DP - although you could opt to allow for in-between values (that $100,000 gemstone is 13% of the way between $70k - 100 DP - and $300k - 200 DP - so 113 DP would be appropriate). Optionally, using the item as the "housing" for a Charm counts it as though it were worth more for purposes of Decanic trappings; probably not a whole category, but some boost may be appropriate. Because under the current rules 300 DP would be a 100% discount, that would need expansion as well. Something like this might work (note it's identical to what's in
GURPS Thaumatology: Ritual Path Magic up to 50%/100 DP):
Code:
% DP
5 5
10 10
15 15
20 20
25 30
30 40
35 50
40 60
45 80
50 100
55 150
60 250
65 500
70 1,000
75 2,000
80 5,000
85 10,000
90 20,000
95 50,000
This should be easy to extrapolate in-between values, and note 95% is the greatest possible discount, it doesn't continue beyond this. Getting to that point would require sacrificing a single object worth $7 billion or more (or using a $15 billion object as a focus). That's around 5 million months - over 400,000 years - of Average income, so one would certainly expect a pretty serious ritual.